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6 

DE    PONTIBUS: 


A    POCKET-BOOK 


FOK 


BRIDGE    ENGINEERS. 


BY 

J.   A.    L.    WADDELL, 

C.E.,  U.A.So.,  Ma.E.; 
Knhiht  Comummler  of  the  Jopnnese  Order  of  the  Kisiug  Sun:   Con- 
sulting Engineer,  Kanma  City,  M".;  Memher  of  the  American 
Society  of  Civu  iLiigineers;  of  La  Societe  den  Ingenieurs 
Civils   Paris:  of  the  Rensselaer  Society  of  Engin- 
eers and  the  Society  for  the  Promotion  of  En- 
gineering Education ;  and  Honorary 
Member  of  the  Kogaku  Kyokai 
(Japanese  Engineer- 
ing Society.) 


FIRST   EDITION. 

FlUST   THOUSAND. 


NEW   YORK: 

JOHN  WILEY  &   SONS. 

London      CHAPMAN  &   HALL,    Limited. 

1898. 


Copyright,  1898, 

BY 

J.  A.   L.   WADDELL. 


BonitRT  nnnMMONP.  klkctuottpkr  A^p  phintkr,  ^f.w  vorb. 


To 

iUc(5iU  Umocrsitn, 

tlie  representative  Institution  of  learning  of  the 

Dominion  of  Canada 

{the  mitfior's  native  country), 

this  little  treatise  is 

DEDICATED 

as  a  mark  of  the  author's  grateful  appreciation 

of  the  distinction  accorded  him 

by  that  university 

in  188?. 

in  conferring  upon  him  ttco  engineering  degrees. 


1;)L074 


PREFACE. 


In  presenting;  to  the  public  a  new  teclinicul  work,  it  is  the 
rtistoiii  to  offer  some  sort  of  apology  for  its  appearance  ;  hence 
tile  aullior  of  lliis  treatise,  in  order  not  to  be  C(»ii8itlere(i  pecu- 
liar, feels  it  incunil)ent  upon  liiui  to  do  liltewise.  Moreover, 
tliurtj  is  in  this  case  a  good  reason  for  beginning  his  book  with 
an  apology,  l)ccau8e  of  his  audacity  in  imposing  upon  the 
good  nature  of  the  engineering  ))rofessiou  by  asking  its  mein- 
1)6 IS  to  read  still  another  work  iipon  tlie  already  overwritten 
subject  of  bridges.  If  he  cmild  do  so,  the  author  would 
here  plead  primuin  tnnpns;  but  this  is  l)y  no  means  liis  first 
ollente.  Perhaps,  liioiigh,  the  fact  liiat  it  is  twelve  years  since 
tlie  appe.-irance  of  his  last  book  (p.implilets,  of  couise,  ex- 
cepted) will  be  considered  by  his  critics  as'a  "mitigating  cir- 
cumstance" in  this  case. 

But,  to  speak  seriously,  if  this  work  were  a  mere  rehash  of 
otjjer  boolcs,  or  if  It  dealt  witlj  tlie  same  old,  worn-;)Ut  sub- 
jects, the  author  would  not  presume  to  present  it  to  the 
engineering  profession  ;  but,  on  the  contrary,  in  writing  it  he 
lias  endeavored  to  nuike  the  contents  as  original  as  possible, 
and  to  treat  essentially  of  the  finidamental  principles  of 
bridge-designing  and  their  application.  It  will  be  noticed 
throughout  the  book  that  quotations  from  other  works  on 
bridges  are  "conspicuous  by  their  absence,"  and  that  the 
autlior  has  drawn  almost  entirely  upon  liis  own  ])ri>fessioual 
practice  for  examples  to  ilhistrale  Ids  text.  For  the  latter  no 
a|>ology  is  recjuired,  because  his  own  designs  (as  far  as  the 
process  of  development  lias  permitted)  have  naturally  been 
made  in  conformity  with  the  principles  which  he  herein  offers 
as  a  guide  to  bridge-designing  ;  and  tljey  are  therefore  more 
appropriate  as  illustrations  than  the  designs  of  others. 


VI 


pkkfa(;r. 


The  author  desires  it  lo  br  disliiiclly  iiiKU'istood  jii  the  out- 
set tliiit  lif  by  no  iiu-titis  claiiuH  that  Uk.*  nietlioils  of  <i<'«i<rtiitu^ 
niul  coiislrurlioii  <x\\cn  lieieiii  arc  all  either  nriojnal  with  luin 
or  are  the  only  correct  nieliiods  ;  they  arc,  iMwcver,  liie  very 
best  of  whicli  he  IvMows,  wlictlicr  lliey  originated  in  iiis  prac- 
ti(X'  or  were  adopted  in  wljule  or  iu  part  from  llie  practlec  of 
others. 

Probably  tlie  first  point  in  connection  with  tliis  l)ook  whicli 
each  reader  will  find  to  criticise  is  its  peculiar  title.  Each 
will  probably  remark,  "  Why,  in  the  name  of  common  sense, 
did  the  author  choos(;  such  an  indetinile  and  outlandish  title 
as  '  I)e  Pontibus'?"  Header,  its  indeliniteness  is  its  most 
praiseworthy  feature  ;  for  the  work  is  certainl}'  not  a  complete 
treatise  on  bridges,  being  eminently  lacking  iu  illustrations  of 
details,  and  entirel}'  witho\it  any  treatment  of  the  theory  of 
stresses  ;  and  what  title  coidd  be  more  appropriate  to  such  a 
b(X)k  than  the  indetiiute  one,  "  Concerning  Bridges"  ?  But, 
the  captious  reader  will  reply,  "  Why  revert  to  the  Latin 
language?  Is  not  English  good  enough?"  Certainly  it  is;  but 
the  author  had  a  sound  reason  for  usiug  the  Latin,  which  he 
will  proceed  to  explain,  as  the  said  captious  reader  will 
assuredly  not  be  satisfied  without  some  explanation. 

For  five  consecutive  years  of  his  early  life  the  author  de- 
voted more  than  half  of  his  working  time  tc  the  siudy  of  the 
Latin  language  ;  and  this  is  the  first  opportunity  which  has 
occurred  during  the  twenty-two  years  of  his  professional 
career  to  put  the  knowledge  (?)  so  obtained  to  any  practical 
use.  Moreover,  he  fears  that,  even  if  he  be  so  fortunate  as  to 
be  able  to  practise  his  profession  n?'olher  twenty-two  years, 
no  other  occasion  will  occur  to  use  it,  so  he  feels  the  necessity 
for  grasping  this  unique  opportunity  of  a  lifetime. 

Captious  reader,  are  you  now  satisfied  ? 

That  many  readers  will  have  faults  to  And  with  the  book 
goes  without  saying.  Some  may  object  to  its  incompleteness* 
in  that  it  does  not  treat  of  stresses  and  that  it  gives  principles 
without  actual  examples  of  their  application.  To  these  the 
author  would  reply  that  any  information  desire(i  concerning 
the  subject  of  stresses  can  be  found  in  such  standard  works  oq 


rilKPACK. 


fii 


l)i'i(l<r(>8  Hs  tliosc  of  Profs.  Kurr,  Dii  BoIh,  and  Jolinson,  and 
lliul,  if  he  were  lu  nttL'iii|)l  to  illustrulc  tUe  principles  by 
ac'lmil  exiiniplcs  of  designing,  his  book  would  never  be 
tinislied. 

As  staled  in  Clmpters  XI  and  XIX,  the  second  edition  of 
tlie  author's  "General  S|)eclttctitions  for  Highway  Bridges  of 
Iron  and  Steel"  and  the  Mist  edition  of  his  "Coniproniise 
Stniidiird  System  of  Live  Loads  for  Hallway  Hi  idgcs  and  the 
Kquivalenls  for  Saini; "  are  now  exhausted,  and  will  not  be 
reprinted,  as  this  treatise  will  replace  them. 

In  writing  Chapters  XV.  XVI,  XVII.  and  XVIII  it  was 
found  necessary  to  copy  cerlain  portions  of  Chapter  XIV  in 
order  to  make  the  various  specilications  complete;  but  the 
amount  of  repetition  was  made  as  small  as  possible  by 
referring,  wherever  no  changes  were  introduced,  whole  sec- 
tions of  one  set  of  speciiicalions  to  the  corresponding  sections 
in  a  preceding  set. 

The  subject  of  suspension  bridges  is  not  dealt  with  in  this 
work,  partly  because  until  lately  the  author  has  not  paid 
much  attention  to  this  class  of  struct uies,  and  partly  because 
they  are  so  dilTerent  from  other  bridges,  being  suitable  for 
very  long  spans  only,  that  each  suspension  bridge  requires 
special  specltications  of  its  own. 

The  author  has  the  presumption  to  hope  that  there  will  be 
considerable  demand  for  this  book,  for  he  considers  that  it 
will  be  useful  to  the  following  classes  of  readers :  first,  to 
praiaising  bridge-engineers,  because  of  many  little  suggestions 
that  will  help  them  to  effect  improvements  and  to  avoid  mis- 
takes ;  second,  to  young  engineers  in  oflices  of  bridge  s\)e- 
cialists  and  of  bridge-manufacturing  companies,  for  perfecting 
them  in  their  work  ;  third,  to  professors  of  civil  engineering, 
to  show  them  the  pmctical  side  of  bridge-designing  and 
building,  and  to  aid  them  in  giving  their  lectures  on  bridges  ; 
fourth,  to  students  of  civil  engineering,  as  a  supplementary 
text-l)ook  that  will  enable  them  to  understand  the  application 
of  what  they  have  learned  during  their  course  in  bridges; 
fifth,  to  railroad  engineers,  because  of  the  bridge  specltications 
contained,  and  to  instil  into  their  minds  tbe  importance  of 


VIII 


PKKFACK. 


having  their  bridges  properly  dosignod,  tnnniifiictured,  in- 
spected, shipped,  and  erected  ;  and,  sixth,  to  ti  few  county 
coininissioiiers,  wlio  may  (icHire  to  obtuin  througli  the  spccili- 
cations  good  liighway  bridges  at  mininiiiin  Icgititniite  cost. 

The  autlior  lias  endeavored  to  nial(e  the  various  speciflcn- 
tions  in  this  boolc  tliorough,  correct,  and  (;oinpIclc.  If  he  lias 
failed  to  do  so  in  any  particular,  lie  woidd  feel  deeply  in- 
debted to  r.iiy  one  wlio  will  point  out  to  him  how  and  wliero  ; 
and  he  would  be  grateful  to  any  reader  who  will  ill^llul  lum 
of  any  typographical  or  other  errors  that  he  may  discover  , 
for  all  errors  found  in  the  tirst  edition  will  be  corrected  in  liie 
second,  provided  tlie  work  l»e  well  enough  received  i)y  the 
profession  to  warrant  the  issue  of  another  edition. 

In  conclusion  the  author  desires  to  acknowledge  with  many 
thanks  his  indebtedness  to  bis  assistant  engineers,  Ira  O. 
Hedrick,  Assoc.  M.  Am.  Soc.  C  E. ;  I^ee  Treadwell,  M.  Am. 
Soc.  C.  E. ;  and  John  L.  Harrington,  Jun.  Am.  8oc.  C.  E., 
for  valuable  aid  rendered  him  in  the  preparation  and  checking 
of  the  MS,  of  this  work. 


Kansas  City,  Mo„  Oct.  18,  1897. 


CONTENTS. 


r-HAPTKR  PAQB 

I.  Introduction .  .  ...  1 

II.  First  Principles  of  r)<'Hi>riiiiiK   Vi 

III  Ti-ii«  Kconomy  in  I)esij{n   . .30 

IV.  .Kstlietics  in  Design 30 

V.  ('untile ver  Hri(iK<'8.    5,5 

VI.  Ardies 79 

VII.  Tre.stlesai;.;  Viutliicts 80 

VIII.  Klovateii  Riiilroad.s 91 

IX.  M()Vftl)U' Bridtfes  In  (lenernl UW 

X.  FU'volvinj?  DravvhridKe* 119 

XI.  lliKliwiiy  Bridges 130 

XII.  Combined  Bridges     . 133 

XIII.  DetailinK ]:18 

XIV.  Oeneral   Specifications  Ooverninj;    the    Designing    of   Steel 

Railroad    Hridpcs  and   Viaducts  anil  the  Siiperstnictuie 

of  Elevated  Railroads HI 

XV.  Specifications  for  Railroad  Draw  Spans    .  .     ItW 

XVI.  General   Specifications    {;overidng    the    Designing  of    Steel 

Higliway  Uridtces  iind  Viaducts.     ..    . .  iit-i 

XVII.  Specifications  for  HiKhway  Draw-Sptns     .  •^.l' 

XVIII    Ueueral    Sjwcillcations   (Jovernin>r  tlie  jVlaniifacture,   Ship- 
ment, and   Kre«;tion   of  Steel  Bn  Iges,  Trehtles,  Viaducts, 

and  Elevated  Railroads 34.) 

XIX.  The   Compromise  Standard  System  of  Live  Loads  for  Rail 

way  Bridges  and  the  Kqui valenta  for  Same '.JOS 

XX.  Timber  Trestles in 

XXI.  Inspection  of  Materials  and  Workmanship  'i6\ 

XXII.  Designing  of  Piers 301 

XXIII.  Triangulatlon 317 

XXIV.  Office  Practice ...328 

TABUts 349 

Index 373 

u 


LIST  OF  TABLES. 


TAll 


TABLE  PAGE 

400 

I.  Coefficients  of  Impact  for  Railway  Bridges,  /  =  ,    .   ,^    .  .  351 

L  +  nOO 

100 
II.  Coefficients  of  Impact  for  Highway  Bridges,  '=,.,,„•    -352 

80/ 

III.  Intensities  for  Inclined  End  Posts,  P  -  18000  -   - SM 

r 

IV.  Intensities  for  Top-Cliord  Compression  members, 

P  ^  18000  -  — 354 

r 

V.  Intensities  for  Intermediate  Posts  and  Subdiagonals, 

us)} 

P=  10000  -   - 8.5.') 

r 

Intensities  for  Columns  of  Viaducts  and  Elevated  itailroads, 

00/ 
and  for  all  Lateral  Struts,  /'  ^  16000  -    - :«6 

Centrifugal  Force  in  iVrcenlages  of  Live  Load     .  .   ,     ',Vu 

Sizes  and  Weights  of  Stay-I'lates  and  Lacing-Bars  for  Ordi 

nary  I'osts ,.    3.58 

Bending  Moments  on  Pins. 361 

Bearing  on  Pins ... 362 

Intensitie.s  for  Forlted  Ends  and  Extension-Plates  of  Ci-m- 

pression-Members,  P  =  10000  -  ^p' 363 

Shearing  and  Bearing  Values  of  Rivets 364 

Coefficients  of  U'tantf  for  both  Compression  and  Tension 
Stresses  in  Bottom  Chords  of  Through-Bridges  and  Top 
Chords  of  D»'ok-Bridges,  due  to  Wind  Loads  applied  to  s^aid 
Chords,  when  the  Lateral  System  is  of  Double  Cancella- 
tion   36R 

IV  sec  9 

XrV.  Coefficients  of  • (where   71  =  Number  of  Panels   in 

n 

Span)  for  Wind-Load  Stresses  in  the  Diagonals  of  Lateral 

Systems  of  Single  Cancellation.    These  Coetnclents  apply 

X 


VI. 


VII. 
VIIL 

IX. 

X, 

XI. 


XII 
XIll. 


LIST   OF  TAI5LKS. 


xi 


TABLE 


XV. 


PAOK 

tfi   Lateral  Systems  C()iim><>s«'(1   of  Intersecting  Diagonal 

UodsorofSiiiKi  ■  I'iagoniil  Struts a66 

Coetllcients  of  U' tan  fl  foi-  » '.unpression-Stresses  iu  Wind- 
ward Bottom  Chords  uf  Throiigii  Bridges,  und  Windward 
Top  Chords  of  I)eck-Brid^'e»,  one  to  Wind  Loads  apphed 
directly  to  said  Chords,  when  the  Lateral  System  is  of 
Single  CaneelUitiun.  The  Tensile  Stresses  in  Leeward 
Chords  are  iiuiuerically  Kqual  to  the  Compression  Stresses 

given  in  Table  for  One  Panel  nearer  End  of  Span 367 

Intensities  of  Working  Stresses  foi  Vnfit-ns  Materials 368 

Maximnni   Stresses  imder    Dead   and   Live   Loads  in  Pratt 

Trusses 'J'j' 

XVUl.  Superelevations  of  Outer  Rail  on  Curves...  374 


XVI. 
XVII. 


''V 


LIST   OF   PLATES  OF  CURVES  AND 
DIAGKAMS. 


PLATE 

1.  Axle  Concentrations  for  tlie  Conipronuse  Sta.ulard  System  of  Live 

Loads  for  Railway  Biitlges. 
11,  DiaKrani  of  Live  Load  End  Shears  for  Raihviiy  Bridges 
111.  DiaKiain  of  E<|iiivaleiil  Live  Loads  for  Railway  Plate  Girders. 
IV    Diagram  of  Equivalent  Live  Loads  for  Trusses  of  Railway  Bridges 
V.  Diasnuu  of  Live  Loads  per  Square  Foot  of   Floor  for  Highway 

Bridges. 
VI.  Diagram  of  Equivalent  Live  Loads  from  Electric  Cars  on  Highway 

Bridges. 
VH.  Diagram  of  Wind  Loads  for  Railway  Bridges, 
Vni    Diagram  of  Wind  Loads  for  Highway  Bridges. 
IX.  Diagram  of  Reactions  for  Balanced  Loads  on  Draw  Spans. 
X.  Diagram  of  Weights  of  Metal  in  Trusses  of  Cantilever  Bridges. 

xii 


DE  PONTIBUS. 


ClIAPTKR  I 


INTRODUCTION. 


Winr.E  it  is  true  tlmt  the  devulopment  of  bridge-building 
ill  America  owes  much  to  the  syst«'m  so  long  in  vog»«  of 
bidding  on  ronipetilive  plans,  in  tliat  sucii  competition  has 
tended  to  sharpen  the  wits  of  the  engineers  of  tlie  competing 
companies,  it  is  equally  true  that  the  sjiid  competition  lias  done 
all  the  good  it  can  for  the  sciouce  of  bridge-designing,  and 
now  acts  as  a  clog  to  prevent  its  further  tidvancemcnt.  The 
correctness  of  this  assertion  .scarcely  needs  any  demon.stratioii, 
but  it  may  be  well,  notwitlistanding,  to  give  here  a  few  rea-sons 
therefor. 

As  human  nature  is  the  .same  the  world  over,  and  as  men  in 
general  are  working  for  the  almighty  dollar,  it  stands  to  rea- 
son that  when  a  bridge-company's  engineer  is  preparing  upon 
fi.xed  specifications  a  design  to  be  used  as  a  basis  for  a  competi- 
tive bid,  and  when  he  knows  that  in  nineteen  cases  out  of 
twenty  the  contract  will  be  awarded  to  the  lowest  bidder 
whose  design  conforms  to  the  letter  of  the  specitications, 
although  it  may  not  be  up  to  the  requirements  of  good 
engineering  practice,  he  will  take  advantage  of  every  weak 
point  and  omission  in  said  specitications,  even  if  his  engineer's 
conscience  proclaim  the  design  he  .submits  to  be  worse  liiaii 
faulty.  As  it  is  enlirely  pnicticjible  to  take  advantage  of  any 
set  of  railroad-bridge  specilications  yet  published,  it  is  evident, 


l)K    I'OXTIBUS. 


that  as  long  as  conipftitivo,  lump-sum  bids  arc  the  f.ishion, 
just  so  long  will  railroatl  bridges  be  badly  designed.  As  for 
highway  bridges,  their  letting,  ilesigning,  and  construction 
are  so  often  left  in  tlie  hands  of  such  incompetent  and  unseru 
pulous  parties  that,  until  some  fundamental  cliauge  in  existing 
conditions  be  eltecled,  notiiing  can  be  done  to  improve  the 
present  imscieiititic,  wretciied,  and  even  crimiual  methods  of 
highway-bridge  building. 

Concerning  llie  prejudicial  eirect  of  competitive  designing 
upon  tlie  development  of  the  science  of  bridgc-engiru'cring, 
the  author  can  spctak  autlioritatively,  l)ecause  for  about  six 
years  he  acted  as  engineer  to  a  bridge  company  During  that 
time  he  lost  many  contracts  for  small  bridges  because  lie 
insisted  on  incorporating  in  his  designs  certain  features  re 
(pnring  extra  metal,  which  features  he  considered  essential, 
although  they  were  not  called  for  in  the  specitications.  On 
the  other  hand,  he  once  earned  a  conunission  of  more  than 
ten  thousaiul  dollars  upon  a  single  piece  of  work  by  knowing 
how  to  lake  the  greatest  advaniage  of  the  specitications  up>)n 
which  bids  were  recpiested.  In  defence  of  this  action,  iiow- 
ever,  it  must  be  mentioned  that  it  was  undersio  .d  at  the  outset 
that,  after  tlie  seleciiou  of  the  successful  competitor,  the 
contract  was  to  be  adjusted  upou  the  basis  of  a  pound  pri(;e 
for  the  metal-work.  liy  reason  o''  this  feature,  the  autiior 
was  able  to  correct  later  on  all  tiie  weak  points  of  his  prelimi- 
nary design  to  such  an  extent  that  the  structure  until  within  " 
few  years  was  by  far  the  best  of  its  kiLi.  .,uilt. 

Tins  case  is  given  merely  as  an  illustration  of  how  great  are 
the  possibilities  for  trimming  a  design  wiiich  is  based  upou 
onlinary  standard  spe<;iiications,  and  how  great  is  the  tempta- 
tion to  take  advantage  thereof.  Anotiur  way  to  illustrate 
this  point  is  to  compare  the  weights  of  the  structures  manu- 
factured and  built  by  any  bridge  eoinpauy  by  the  lump  sum 
with  the  weights  of  similar  structures  manufactured  and  built 
by  the  same  company  for  a  pound  i)rice.  'J'iu!  ditterenct^  in 
weight  often  runs  as  high  as  fifteen  or  twenty  per  cent  or  even 
higher,  if  there  be  no  supervising  engineer  to  hold  the  bridge 
company  in  check. 


^^ 


|i 


I  NT  UO  DUCT  ION. 


3 


iisbion. 

As  for 

ruction 

miscru 
xisliiig 

ovu  the 
ukIs  of 


i)g  that 


For  a  raihviiy  conipany,  the  most  satisfactory  method  of 
building  bridges  is  cither  to  have  a  pcrniaueut,  competent 
bridge  engineer  in  its  employ,  or  to  relaiti  some  specialist  of 
established  reputation  to  i)rep:ire  specifications  and  complete 
detiiilcd  plans  (not  working  drawings,  however)  for  all  ils 
bridges,  and  to  provide  competent  inspectors  to  see  that 
during  manufacture,  shipment,  and  erectiou  the  plans  and 
specifications  are  strictly  followed. 

Tlio  necessity  for  the  specialist  to  stand  between  the  pur- 
chaser and  the  manufacturer  of  structural  steel  is,  as  a  general 
rule,  not  appreciated  l)y  the  purchaser,  \inles3  ho  has  already 
had  some  experience  in  letting  contracts  for  and  in  the  building 
of  steel  structures  without  engineering  aid  in  the  designing 
and  supervision.  When  the  purchaser  puts  himself  in  the 
hunds  of  the  manufacturer,  he  is  pretty  sure  to  get  tlie  worst 
of  it  ;  for  if  the  contract  be  let  by  schedule  prices  the  struc- 
tuie  is  lial)le  to  be  loaded  down  with  useless  metal,  wldle  if 
the  contrac't  be  let  ft)r  a  lump  sum  the  structure  will  probably 
he  ruined  by  having  the  metal  "  skinned  "  out  of  it,  especially 
in  the  most  important  parts,  viz.,  tlie  details.  Moreover,  the 
manufacturer  is  seldom  capable  of  evolving  a  truly  lirst-class 
design,  for  the  reason  that  his  training  has  always  been  in  the 
line  of  his  own  pecuniary  interests,  which  are  to  obtain  the 
ma.vimum  of  pay  for  the  minimum  of  structure  ;  so  that,  even 
when  given  all  the  metal  and  money  that  ho  could  ask  for 
when  preparing  a  design,  he  would  not  succeed  in  making  a 
really  good  one,  simpl}-  becauNO  of  not  knowing  h(>w. 

On  the  other  haiul,  the  specialist  should  stand  between  the 
contractor  and  th(!  purchaser,  so  as  to  see  that  the  latter  does 
not  take  any  undue  advantage  of  the  former  by  means  of  a 
harsh  or  unjust  interpretation  of  the  si)ecitications,  especially 
when  the  contractor  has  suffered  loss  or  delay  on  account  of 
causes  beyond  his  control. 

Occasionally  a  .nanufaeturer  offers  to  prepare  the  plans  for 
certain  portions  (if  the  work  on  the  plea  that  he  has  had  so 
much  more  e-xperionce  iti  such  matters  than  the  engineer.  The 
accei)li.nce  of  this  o!!er  by  either  the  purchaser  or  theengineer 
is  a  nusiake  ;  for  the  engineer,  if  he  have  sufficient  ability  to 


DK    1'0NTIBU8. 


warrant  liis  being  rotaincd  on  ihe  work,  can  by  careful  study 
almost  always  evolve  a  belier  design  than  can  tlie  contractor, 
even  if  it  be  the  lirst  experience  wliidi  the  former  lias  had  in 
connection  witii  the  portion  of  the  work  under  consiileration. 
On  two  or  three  occasions  only,  and  several  years  ago,  the 
author  was  either  induced  or  conipelleil  by  the  purchaser  to 
defer  to  the  greater  experience  of  the  contracting  engineer  ; 
aud  in  each  case  he  lias  had  reason  to  regret  the  concession  ; 
so  he  has  conchidcd  that  in  future  he  will  receive  with  thanks 
any  suggestions  which  the  manufacturer  may  ofl'er,  give  them 
due  cousideraliou,  and  then  make  the  design  as  he  liimself 
sees  fit. 

Considerable  opposition  to  the  methods  of  design  advanced 
in  this  work  and  to  the  specilications  given  is  anticipateil,  on 
the  plea  that  the  requirements  are  too  exacting  and  llial  the 
class  of  work  called  fcr  is  unnecessarily  refined  and  conse- 
quently expensive.  To  such  opposition  the  author  would  reply 
as  follows  :  First,  the  designing  and  ])uilding  of  bridges  and 
similar  structures  cannot  be  too  well  or  carefully  done  ;  and, 
second,  that  within  the  last  three  years,  upon  some  fifty  or 
sixty  thousand  tons  of  the  author's  work  designed  in  accord- 
ance with  the  said  methods  and  built  in  accordance  witli  the 
said  specifications,  the  prices  quoted  by  the  competing  man- 
ufacturing companies  were  extraordinarily  low,  and  tluit  no 
complaint  of  any  account  has  since  been  raised  by  the  manu- 
facturers in  respect  to  tlie  expense  involved  by  either  the  de- 
signs or  the  specifications. 

The  principles  of  design  given  in  the  succeeding  chapter 
should  be  adhered  to  in  all  structural  metal-work  ;  and  any 
violation  of  any  one  of  them  is  a  mistake  that  will  be  regretted 
sooner  or  later  by  the  parties  owning  the  structure.  Many  of 
these  principles  are  violated  constantly  by  shop  draftsmen, 
even  when  the  engineer's  drawings  s!)0W  the  details  correctly. 
This  is  due  partly  lo  custom  in  designing  certain  details  in 
certain  ways,  aud  partly  to  the  ignorance  of  the  draftsmen. 
The  author  would  urge  upon  young  engineers  who  are  work- 
ing on  plans  for  structural  metal-work  to  adhere  to  the  prin- 
ciples herein  given  whenever  it  is  practicable  for  them  to  (Jp 


INTKODUCTIO.V.  D 

so.  Had  more  attention  been  paid  to  first  principles  of  de- 
sign when  tlie  plans  for  most  of  the  New  Yorii  and  Brooklyn 
elevated  railroads  were  beitii;  jirepnred,  millions  of  dollars 
would  have  been  saved.  This  statement  can  be  verilied  ijy  u 
perusal  of  the  author's  paper  on  Elevated  liiiilroads,  referred 
to  and  quoted  from  in  Chapter  VIII,  more  especially  the 
resume  of  the  discussions  and  Mr,  Iledrick's  report  on  tiie 
said  New  York  and  Brooklyn  elevated  railroads. 

In  spile  of  all  tlie  care  that  the  most  expert  desiguer  can 
give  to  his  work,  errors  of  greater  or  less  magnitude  will  creep 
in  occasionally;  and  one  can  always  improve  somewhat  upon 
any  tinished  design.  Such  being  the  case,  it  follows  that  the 
designing  of  steel  structures  should  be  intrusted  to  expert 
and  disinterested  designers  only,  instead  of,  as  is  generally  the 
case,  to  the  cheap  draftsmen  employed  by  the  manufacturing 
companies. 

Tiiere  are  a  few  features  of  the  specifications  given  in  Chap- 
ters XIV.  and  XVIII.  vvliich  will  require  a  little  explanation 
or  comment.  This  will  be  given  in  this  chapter,  the  various 
items  being  treated,  as  nearly  as  may  be,  in  the  order  in  which 
they  occur  in  the  said  specifications. 

"A"  Truss  Bridges. 


This  style  of  structure,  originated  by  the  author  and  covered 
by  letters  patent,  is  a  four-panel  truss-bridge  having  eye-bars 
in  bottom  chords  and  centre  verticals,  and  rigid  members  for 
all  the  other  portions  of  the  trusses  and  for  the  entire  lateral 
system.  It  was  evolved  in  this  way;  P'or  a  number  of  years 
the  author  was  dissatisfied  with  all  railroad  bridges  for  spans 
between  the  superior  limit  of  the  plate-girder  and  a  length  of 
about  one  luindred  and  fifty  feet,  ordinary  piu-connectcd, 
through,  Pratt  truss(!S  being  too  light  and  vibratory,  and  the 
riveted  bridges  as  then  built  being  clumsy,  unscientific,  and 
imeconomical.  On  this  account  he  tried  for  some  time  to  find 
an  opportunity  to  experiment  upon  a  design  of  his  own  to  till 
a  portion  of  the  gap,  but  the  opportunity  did  not  occur  until 
April  1893,  when  he  was  retained  by  the  General  Manager  of 


6 


DK  I'ONriHifg. 


the  Kansiis  City,  Piltsbiiru;,  jiiid  Quif  Ruilrotul  Company  to 
design  some  bridges.  Afier  ti  little  porsuasiou  the  General 
jNranuger  was  induced  to  agree  to  hiiilil  a  100-ft.  "A"  truss 
span  as  au  experiiueut;  but  when  Ik;  sjiw  the  completed  i»lans 
he  ordered  at  once  four  bridges  to  be  built  therefrom,  and  tins 
style  of  structure  was  soon  afterwards  adopted  as  the  standard 
100- ft.  span  for  the  road. 

Tliese  bridges  have  shown  such  rigidity  under  tralllc  that 
they  have  been  used  on  the  St.  Louis  Southwestern  Railway, 
and  have  been  adopted  as  the  standard  for  spans  between  si.\ly- 
live  feet  and  one  hundred  and  sixteen  feet  by  the  Nii)pon  Kail- 
way  Company  of  Japan. 

The  advantages  of  this  type  of  bridge  are  great  rigidity  in 
all  directions,  ease  and  clieai)ness  of  erection,  and  economy  of 
metal  when  it  is  compared  with  structures  of  other  types  hav- 
ing equal  stnength  and  i  igidity. 

IMPACT. 


The  uncertainty  as  to  the  magnitude  of  the  ellcct  of  impact 
on  bridges  has  for  many  years  been  a  stiunl)ling-bl()ck  in  the 
path  of  systenuzation  of  bridge-designing,  and  wMl  continue 
to  be  so  until  some  one  makes  an  exhaustive  series  of  experi- 
ments upon  the  actual  intensities  of  working  stres.ses  on  ail 
main  members  of  modern  bridges  of  the  various  types.  Tlie 
making  of  these  experiments  has  long  been  a  dream  of  tiie 
author's,  and  it  now  looks  as  if  it  would  amount  to  more  than 
a  mere  dream  ;  for  the  reason  that  the  general  manager  of  one 
of  the  principal  Western  railroads  has  agreed  to  join  the  author 
in  the  making  of  a  number  of  such  experiments  on  certain 
bridges  of  the  author's  designing,  the  railroad  company  to 
furnish  the  train  and  all  facilities,  and  the  general  manager 
and  the  author  to  provide  the  ajjparalus  and  experimenters. 
It  is  only  lack  of  time  that  has  prevented  these  experiments 
from  being  made  this  year,  and  it  is  expected  that  they  will 
b(!  finished  in  1898.  It  is  hoped  that  tlifc  result  of  the  experi- 
ments will  be  either  to  determine  a  proper  formida  or  curve 
of   jierceiitages  of    impact   for    r.iilroad    Itridges,   or  else   to 


INTUODL'CTIO.V.  7 

iniiugurate  n  soiit'S  of  fiiiUicr  ox|H'riiiUMils  lliat  will  (Ictenniiie 
it. 

Ml'!iii while  the  author  lias  adopted  temporaiil}'  the  forimila 
givL'ii  ill  Ciiaptcr  XIV.,  vi/,  , 


/  = 


40000 


L  -f-  oUO' 


in  whi(  h  1  U  the  percentage  for  impact  to  he  added  to  the  live 
I' iid,  and  L  is  the  length  in  feet  of  span  or  portion  of  span 
iliat  is  covered  hy  the  said  load. 

This  foiinula  was  estahlished  to  suit  the  average  practice  of 
lialf  a  dozen  of  the  leading  bridge  engineers  of  the  United 
States,  as  given  in  their  standard  specitlcations,  and  not  because 
I  he  author  considers  tliat  u  will  give  truly  correct  percentages 
tor  iinpiict. 

In  spite  of  ail  that  has  Iwjcn  said  to  the  contrary  in  the  past 
or  that  may  be  said  in  the  future,  tlie  impact  method  of 
proportioning  bridges  is  the  only  rational  and  scientifically 
practical  metliod  of  designing,  even  if  the  amounts  of  impact 
assumed  be  not  aljsolulely  correct ;  for  the  method  carries  the 
effect  of  impact  into  every  detail  and  group  of  rivets,  instead 
of  merely  ailecting  the  sections  of  the  main  members,  as  do 
tlie  other  methods  in  common  u.sc. 

The  assumption  madi;  in  some  specifications  that  the  live 
lo:id  is  always  twice  as  important  and  destructive  as  the  dead 
load,  irrespective  of  whether  tlie  member  considered  be  a 
panel  suspender  or  a  bottom  chord-bai  in  a  flve-hunciii,cl  foot 
span,  is  absurd,  and  involves  far  greatei  errors  than  those  tliat 
would  be  caused  by  any  incorrectness  in  tlie  assumed  impact 
formula. 

The  author  acknowledges  that  he  anticipates  finding  the 
values  given  by  tli'  formula  somewhat  high  ;  but  it  must  be 
remembered  tliat  the  s.iid  formula  is  intended  to  cover  in  a 
general  way,  also,  the  ellVcts  of  small  variations  from  correct- 
ness in  shop-work,  or  to  provide  for  what  the  noted  bridge 
engineer,  tlie  late  C.  Slialer  Smilii,  used  to  term  tlie  factor  of 


8 


i)F  i'()N"riM(r.s. 


Using  n  uniform  tension  intensity  of  18,000  lbs.  for  eyc-lmrs 
nii<l  1(),000  lbs.  for  built  inenibcis  will  strain  the  nietul  up  to 
nc'iirly  one  liiilf  of  tlie  eiaslic  limit  siiown  l)y  specimen  tests, 
and  prob!il)ly  somewiiiil  liigiicr  lliaii  owv  half  of  same  sliown 
by  tests  of  full-si/e  meml)ers.  So  long  us  the  greatest  actual 
intensity  of  worliing  stress  is  Itept  in  the  neighborhood  of  one 
half  of  the  elastic  limit,  sutlieient  precaution  lias  been  talien 
against  all  possil)ility  of  failure  by  load  even  in  the  far-distant 
future. 

The  impact  formula  for  highway  bridges  given  in  Cluipter 
XVI.,  viz., 

10000^ 
~  L  +  f  50" 

was  established  to  lit  the  author's  practice.     Its  correctness  is 
not  likely  to  be  ever  determined  by  experiment. 

MKDIUM   HTEKL. 

The  reason  for  using  this  metal  almost  exclusively  and 
barring  out  .soft  steel,  except  for  rivets  and  adjustable  mem- 
l)ers,  is  because  the  two  kinds  of  the  raw  material  cost  almost 
exactly  the  same  i)or  pound,  while  medium  steel  is  the 
stronger  of  the  two  by  fully  fifteen  per  cent,  and  is  in  every 
particubir  just,  as  reliable  and  satisfactor}'  for  use  as  soft  steel. 
The  only  advantage  claimed  for  the  latter  is  that  it  re(iuire8 
no  reaming,  which,  in  the  author's  opinion,  is  a  fallacy,  be- 
cause, in  order  to  obtain  proper  matching  ot  rivet-holes  in  the 
various  component  parts  of  a  piece,  reaming  is  a  siiie  qua  lutn. 


HIGH   8TKEL. 

The  use  of  this  material  is  limited  to  those  portions  of  very 
long  spans  where  the  impact  is  small,  and  where  there  is 
neither  reversion  of  stress  nor  any  condition  even  approaching 
same.  The  speciiications  bar  out  its  employment  for  inter- 
mediate posts  of  simple  trusses,  because  in  modern,  long-span 


IKTKOnUCTIONf. 


d 


'^e-I)iirs 
up  to 
tests, 
sliown 
actual 
of  one 
taken 
istuut 

'luipter 


bridges  the  top  rliords  are  K)  curved  tliat  tlie  stresses  in  vertical 
posts  either  reverse  or  vary  beivveun  wide  limits. 


HiinrrNciiiNo  and  hkamino. 

To  inaugurate  the  exciusivn  \ise  of  this  process  for  all 
important  work  in  stniclunil  ste';!,  tlie  author  has  fought  a 
long  and  hitter  fight  with  tl»e  manufacturers,  and  it  begins  to 
look  as  if  lie  were  going  to  win  eventually.  At  any  rate  he 
has  succeeded  in  having  it  adopted  on  all  of  his  own  work  for 
several  years  past  in  sjiite  of  a  most  powerful  adverse  influence 
bioughl  to  bear  upon  the  purchasers  by  certain  of  tlie  largest 
ni  inufaclurers  in  the  United  States.  Again,  in  his  paper  on 
Elevated  llailroails  he  has  advocated  most  unequivocally  the 
adoption  of  subpunchiiig  and  rciiming  on  all  imi)()rtant  metal- 
work,  and  his  views  have  b(!eii  indorst d  l)y  a  majority  of  tlie 
engineers  who  discussed  the  subject.  Any  reader  who  is  in 
doubt  concerning  this  question  is  advised  to  read  all  that  is 
written  on  the  subject  in  the  original  paper,  the  discussions, 
and  the  resume,  all  of  which  have  been  published  in  the  Trans- 
actions  of  the  American  Society  of  Civil  Engineers  for  1897. 


P.VINTING. 

In  respect  to  painting  structural  steel,  engineers  as  a  body 
appear  to  be  unsettled  in  opinion.  Each  one  either  has  a  pet 
paint  of  his  own  or  else  is  experimenting  in  a  hapha/.:ird  way 
to  find  one.  In  the^v'.i^w^'  of  the  aforesaid  paper  on  Elevated 
Uailroads  the  author  wrote  as  follows  in  respect  to  this  matter; 

"  In  short,  the  engineering  profession  is  all  at  sea  on  the 
paint  question,  and  is  likely  to  remain  there  until  there  is 
some  organized  investigation  made.  In  the  author's  opinion 
the  subject  is  one  of  sufticient  importance  to  warrant  the  ap- 
pointment of  a  special  committee  of  the  Society  to  experiment 
and  investigate  on  the  subject  for  a  term  of  years  until  some 
valid  conclusions  can  be  reached  " 

A  short  time  ago  the  author  followed  this  with  a  formal 
proposition  to  the  Society  to  appoint  such  a  committee  ;  but 


10 


UK    I'ONTIIJIS. 


tlu'  iiiolion  uflercuusi(lt'r!il)lo  disciiHsiou  both  j)ro  and  con  una 
lost.  CoiisidiMing  tlie  fuel  timt  tlu;  people  of  the  United  Stiite'* 
arc  invcstiiii^  tiniiiiully  ni.in}'  iiiilliotis  of  (hilltiis  in  .stnictitral 
Ht(  el,  and  tliaf  no  sjitisfaclorv  pn^scrvative  for  tlie  metal  iuis 
yet.  be(!n  found,  one  would  think  that  the  qucHtinn  of  the  best 
kind  of  paint  for  nielul-work  and  the  best  mot  hod  of  paint  inj^ 
woidd  bt!  a  proper  snlijcct  of  investigation  for  a  speelal  com 
nnttie  of  the  American  Society  of  Civil  Engineers. 

Since  this  negative  vote  was  east,  the  technical  papers  Inive 
stated  that  certain  jmrlies  in  New  York  City  have  at  consider- 
able expense  inaugurated  a  series  of  practical  tests  of  ji  nuni- 
l)er  of  brainlsof  mcttdwork  paint.  The  results  thereof  ought 
to  be  of  great  value  to  the  engineering  profession  ;  but  the 
good  work  should  be  carried  still  farther  by  an  authorizid 
body  of  well-known  engineers,  who  would  be  willing  to 
devote  a  portion  of  their  time  for  many  years  to  the  invcstiga- 
tiou. 

A  perusal  of  this  introductory  chapter  may  cause  the  readei- 
to  think  that  the  author  is  at  variance  with  the  manufacturers 
of  structural  steel  ,  but  such  is  by  no  means  the  case,  for  his 
relations  with  them  on  (H)nstruclion  are  almost  invariably  of 
the  most  pleasant  discrlption.  Moreover,  the  assertions  nuule 
iiereln  concerning  tlie  opposition  of  steel  manufacturers  in 
general  to  improvements  in  design  and  in  the  (pndlty  of  the 
manufactured  product  do  not  apply  to  all  of  the  nnmufac- 
turers  of  structural  steel  in  the  United  States  ;  because  there 
are  several  companies  who  are  always  ready  to  do  anything  in 
reason  to  aid  an  engineer  in  making  investigations  and  im- 
provements, and  who  are  continually  putting  in  new  machin- 
ery for  the  purpose  of  bettering  their  output.  It  is  altogether 
natural  tliat  the  manufacturer  should  try  to  make  all  tlie 
money  he  can  by  adopting  simple  details  which  are  easily 
nninufactured  and  easily  put  together  In  the  field,  and  by  avoid- 
ing such  slow  and  tedious  shop  processes  as  subpunching  and 
reandng ;  and  it  is  also  altogether  natural  that  the  consulting 
engineer  should  use  every  endeavor  to  secure  c(.'rtain  results  in 
linished  structures  whicih  are  in  the  Hue  of  improvement  and 
of  ultimate  economy  for  his  employers  :  hence  It  is  to  be  ex- 


iNTKoDirrrrox. 


11 


pcoted  llmt  llie  siiid  iimTiufiicliircrs  and  enginccra  will  orrn. 
sioiiidly  disai^rce,  luid  liiat  each  purty  will  iKtttlc  for  his  sup- 
posed rights.  Tills  is,  liowevcr,  no  reison  for  ill  feeling  or 
for  any  (.onHict  helwi.'en  Ihe  ntanufacturcr  and  the  engineer 
after  the  contract  is  awarded  on  lixed  plans  and  specitieations. 
The  reader  is  advised  to  examine  Uie  various  taMes  appended 
to  th  s  hook  so  as  to  see  if  he  can  iitili/,(!  Iheni  in  hi<  work  and 
ihiH  save  hiuistiif  Huiue  uuuecessury  labor  in  nniking  euuipn- 
lali<»ns. 


CHAPTEIi  II. 


FIRST  PRINCIPLES  OF   DESIONINQ. 


Both  the  student  and  tlio  practitioner  in  bridge-designing 
will  do  well  to  recognize  and  bear  constantly  in  mind  certain 
first  principles  of  design  ;  and  to  enable  them  to  do  so,  tiie 
author  offers  thu  following,  which  he  considers  will  cover  the 
essential  fundamental  principles  that  should  govern  the  de- 
signing of  all  structural  metal-work.  Most  of  these  will  be  re- 
peated in  the  "  General  Specifications  "  given  in  Chapter  XIV. 
under  the  heading  "  General  Principles  in  Designing  all  Struc- 
tures," for  the  reason  that  the  said  specifications  would  be  in 
complete  without  them. 

The  reason  for  this  special  chapter  being  dev'ted   excla 
sively  to  these  general  principles  is  that  the  subject  is  of  the 
utmost  importance,  a?id  needs  much  more  elaboration    tlian 
could  properly  be  given  ii  in  specifications.     On  tills  ac(;ount 
the  statement  of  each  principle  herein  will  be  followed  by  re 
marks  of  an  explanatory  nature  giving  its  raiaon  d'Hre  or  ap 
plication.     It  is  to  be  noticed  that   the  numbering  does  not. 
agree  wilh  that  of  tiie  "  General  Principles  "  in  Chapter  XIV. 

The  attention  of  the  reader  is  called  to  the  fart  that  this 
chapter  is  by  far  the  most  important  one  in  the  book,  in  that 
it  contains  in  a  .oncentrated  form  the  most  important  conclu- 
sions drawn  from  the  author's  entire  experience  in  iiis  chosen 
specially.  The  principles  given  have  been  establiished  nniinly 
by  observation  of  the  mistakes  of  others,  and  in  a  few  cases, 
it  n\ust  be  confessed,  by  those  of  his  own. 

Few  designers  care  to  nud<e  public  their  errors  for  fear  of 
the  result  being  to  their  disadvantage  ;  nevertheless  far  more 
is  learned  from  the  mistakes  of  construction  than  is  learned  in 
any  other  way. 

12 


I 


,] 


FIRST    I'KINCIl'LKS    Ob'    I)KSI(;N1NG 


13 


J 


Tlic  luitlior  would  therefoie  Mug;j;tsl  to  the  render  ihti'  Ik; 
oeruse  t Ids  chapter  carif ally  mow  lliau  once  before  proceed- 
iug  to  the  next. 

Pkincii'M',  I. 

Simplicity  is  one  of  the  highest  attributes  of  good  design- 
ing. 

Il  is  ^^eiHiidly  l)y  means  of  a  wide  (-.xpericnce  oidy  that  the 
young  l)ri(ige  engineer  learns  tlio  trutli  of  this  as.sertion  ;  l)ut. 
Hie  older  lie  grows  and  the  more  knowledge  he  arcjuires  llie 
more  convinced  does  lie  bee  ome  that  simplicity,  not  only  in 
design,  but  also  in  methods  of  execution  of  work,  is  one  of 
the  most  important  (leKulerata. 

Other  things  being  ecpial,  that  design  wiiicli  is  the  most 
simple,  or  contains  tlie  fewest  i)arts,  or  involves  the  easiest 
connections,  is  the  one  which  will  be  preferred  by  competent 
judges 

PrUNCIPLK    II. 

"The  easiest  way's  the  be.st." 

Although  this  principle  was  not  enunciated  originally  in 
relation  to  structural  metal-work,  it  nevertheless  applies  to  it 
just  as  well,  for  the  most  successful  engineer  is  lie  who  in  a 
given  time  can  accomplish  in  a  satisfactory  manner  the  great- 
est amount  of  woi  k. 

This  he  can  do  only  by  the  use  of  every  hibor-saviiig  device 
of  real  value,  by  systemutizing  to  the  greatest  practicable  ex- 
tent all  liiat  he  does,  and  by  making  a  thorough  study  of  true 
economy  of  time  and  labor. 

PlUNCIPI.K  111. 

The  systemization  of  all  that  one  does  in  connection  with 
his  professional  work  is  one  of  the  most  important  steps 
that  can  be  taken  towards  the  attainment  of  success. 

Nor  is  this  by  any  means  all  tlnit  can  be  said  in  favor  of 
estiiblishing  a  thorough  system  of  doing  work;  because,  in  ilie 
first   place,  sucii  a  .s^steiii  enables  one  to  accomplish  a  great 


u 


IJE    PONTIIJUS. 


<leiil  in  a  very  short  tiiiu-,  iiiid,  in  tlic  sccomi  piiice,  it  is  ji  sub- 
jt'ct  of  the  grwUc'sl  s<atisfac:tion  tiud  grutilication  to  the  man  by 
whom  it  was  evolved. 


Pkincii'lic  IV. 

There  is  an  inherent  sense  of  fitness  in  the  mind  of  a 
well-trained  and  well-balanced  metal-work  designer,  which 
sense  of  fitness  is  of  the  greatest  importance  in  all  that  he 
does. 

It  is  this  sense  of  lilness  which  enables  him  often,  when  in- 
specting the  W(jrk  of  oilier  designers,  to  see  at  a  glance  faults 
and  flaws  that  would  escape  the  observation  of  an  untrained 
man.  This  faculty  of  rapid  and  correct  judgment  is  oiw 
which  can  be  developed,  and  one  thai  should  receive  conslanl 
attention  lliroughoul  an  engineer's  entire  career.  It  is  of  spe 
cial  value  in  an  otUce  which  employs  a  large  number  of  drafts- 
men and  computers,  all  of  whose  work  has  to  be  checked  by 
the  head  of  Ihe  office  or  by  a  reliable  assistant.  Nor  is  it  only 
in  connection  with  the  work  of  others  that  this  faculty  is 
valuable,  for  it  is  often  serviceable  to  an  engineer  on  his  own 
personal  work,  perhaps  even  without  his  being  conscious 
thereof,  saving  him  from  making  errors  which  jiure  theory 
might  not  enable  him  to  detect,  or  which  the  aulhorilies  in  liis 
line  have  not  yet  recognized  as  errors.  An  example  of  this 
occiu'red  some  years  ago  in  the  author's  practice  which  will 
serve  to  illustrate  the  i)oint. 

In  |)roportioning  reinforcing  i)iales  at  pinholes,  especially 
for  hinged  ends,  the  author  has  made  a  inactice  of  instructing 
his  draftsmen  to  extend  these  plates  considerably  beyond  the 
length  recpiired  by  tlu;  theoretical  numlKjr  of  rivets  necessary 
for  the  (lonneclion,  without  his  being  able  to  give  any  valid 
or  seieutilic  reason  for  so  doing,  liy  some  experiments  nuide 
upon  the  ullinuite  strength  of  certain  columns  with  hinged 
ends,  the  results  of  which  were  ])ublished  in  the  'J'ntnudc- 
tiouK  of  tl'.e  Engineers'  Sixiely  of  Western  Pennsylvania.  Mr. 
Thonuis  II.  Johnson  has  shown  that  such  pin  plates,  unless 
extended  lieyond  the  length  required  by  the  theoretical  uuui- 


FIRST    I'UINCII'LKS    OF    DESKJN  IN(J. 


If) 


5 


i 


l)er  of  rivfts,  fail  l>efoii!  tlic  full  strciigili  of  the  coiiipression- 
luenibcr  is  diiveloped. 

PUINCIPLE    V. 

There  are  no  bridge  specificationg  yet  written,  and  there 
probably  never  will  be  any,  v/hich  will  enable  an  engineer 
to  make  a  complete  design  for  an  important  bridge  without 
using  his  judgment  to  settle  many  points  which  the  speci- 
fications do  not  properly  cover;  or  as  Mr.  Theodore 
Cooper  puts  it :  "The  most  perfect  system  of  rules  to  insure 
success  must  be  interpreted  upon  the  broad  grounds  of  pro- 
fessional intelligence  and  common  sense." 

At  first  tliougiil  ouc!  iniylit  conclude  Ihut  this  speaks  badly 
for  modern  standard  bridge  speiitications,  and  to  a  certain 
limited  e.xteiit  he  would  be  right  ;  for  while  it  is  (juite  true 
that  no  railway- bridge  specilieatioiis  yet  publisiied  begin  to 
cover  the  entire  ground  of  ordinary'  bridge-designing  at  all 
adequately,  or  nearly  as  thoroughly  as  they  niigiit  readily  be 
made  to  do,  neverlhele.ss  it  is  also  true  that  the  science  of 
bridge-designing  is  such  a  profound  and  intricate  one  that  it 
is  absolutely  ini|>os.slble  in  any  specification  to  cover  the  entire 
field  and  niake  rules  to  govern  the  scientific  i)roportioning  of 
all  parts  of  all  structures. 

The  author  ha.'i  done  his  best  in  Chapters  XIV. -XIX.  of 
this  little  treatise  to  lender  the  last  statemeut  incorrect,  but 
with  what  success  time  alone  can  prove. 

PlfIN<  IIM.K    VI. 

In  every  detail  of  bridge-designing  the  principles  of  true 
economy  must  bo  applied  by  every  one  who  desires  to  be  a 
successful  bridge  engine^ir. 

Tins  subject  is  such  an  important  one  (hat  to  its  cousidera 
tion  the  whole  of  the  ue.vt  chapter  will  be  devoted. 

PitiNfiiM.K  vn. 

In  bridge-designing  rigidity  is  quite  as  important  an  ele- 
ment as  is  mere  strength. 


IG 


I)K    rONTIBUS. 


Ill  fact  each  of  thcst'  properties  is  depenihmt  upon  the 
other,  because  if  a  structure  be  amply  proportioned  in  its 
main  members  for  the  assunud  loads,  but  improperly  sway- 
braced,  the  actual  dynamic  stresses  will  be  greatly  in  excess 
of  the  live-load  stresses  provided  for,  and  the  metal  will  be 
overslruined  in  consequence  ;  while,  <m  the  other  luind,  if 
rigidity  be  provided  for  by  ample  sway-bracing,  but  at  the 
same  lime  the  main  iiieinl)ers  of  the  structure  be  not  ade- 
(piately  proportioned,  the  overstrained  metal  of  the  latter  will 
cause  vibration  to  be  set  up  in  spite  of  the  sulliciency  of  sway- 
bracing.  Both  of  these  faults  are  to  be  found  in  existing 
structures.  The  effect  of  the  lirst  fault  is  usually  tht;  gradual 
wearing  out  of  the  structure  by  impact  and  rack,  and  that  of 
the  second,  the  sudden  collapse  of  the  bridge  without  previous 
warning. 

Pl!IN(  IIM.K    VIII. 

The  strength  of  a  structure  is  measured  by  the  strength 
of  its  weakest  part 

This  statement  is  as  old  as  the  hills,  but  i.s  just  as  valid  to- 
day as  it  ever  was  The  ignoring  of  its  prime  importance  is 
constantly  the  source  of  waste  of  metal  in  structures,  funda- 
me.. tally  weak  in  certain  portions,  liy  increasing  the  weights 
of  other  portions,  and  thus  adding  to  the  total  load  that  the 
weak  parts  have  to  carry, 

PlJlNCIIM.K    IX. 

In  bridge-designing  provision  must  always  be  made  for 
the  effect  of  impact,  either  by  increasing  the  calculated 
total  stresses  by  a  varying  percentage  of  the  live-load 
stresses,  or  by  decreasing  the  intensities  of  w^orking  stresses 
below  those  allowed  for  statically  applied  loads. 

Different  specifications  accomplish  this  result  differently 
The  former  nietluxi  is  undoubtedly  the  more  ."scientific  and 
rational  one,  but  the  latter  is  the  more  common.  The  reason 
for  thi.«  is  that  engineers,  as  a  nilc,  dislike  to  specify  various 
l>ercentages  to  add  to  live  loads  for  impact,  when  such  |)er- 
cent  ages  are   established  entirely  by  guesswork.     An  elabo- 


'i 


FIKST   PRINCIPLES   OF    DESIGNING. 


i: 


■i 


rate  system  of  tests  of  actual  intensities  of  working  stresses 
for  all  main  members  of  modern  steel  bridges  under  live 
loads,  applied  wiih  varying  velocities,  is  probably  more  ur- 
gently needeil  at  the  present  time  by  the  engineering  profes- 
sion tluin  is  any  other  series  of  experiments. 

lu  the  specitications  of  this  treatise  the  effect  of  impact  is 
provided  for,  how  correctly  only  such  experiments  as  those 
just  referred  to  can  demonstrate.  As  pointed  out  in  Chapter 
1.,  the  determination  of  the  various  amounts  of  impact  was 
made  solely  by  adopting  a  few  lixed  intensities  of  working 
stress  and  varying  the  percentages  of  impact  so  as  to  make 
tile  structures  designed  thereby  agree  as  nearly  as  may  bo 
with  the  best  general  practice.  If  the  impact  formulic 
adopted  are  ever  proved  to  be  incorrect,  it  will  be  a  simple 
matter  to  correct  them  in  a  later  edition. 


Principlk  X. 

In  making  the  general  layout  of  any  structure,  due  atten- 
tion should  be  given  to  the  architectural  effect,  even  if  the 
result  be  to  increase  the  cost  somewhat. 

There  is  no  feature  of  bridge-designing  which  has  been  ig- 
nored in  America  to  such  an  extent  as  has  this  ;  and  it  is  only 
of  late  years  that  even  a  few  American  engineers  have  pnid 
any  attention  whatsoever  to  lesthetics  in  that  branch  of  engi- 
neering. The  subject  is  such  an  important  one  that  to  its  con- 
sideration Chapter  IV.  will  be  specially  devoted. 


Principle  XI. 

For  the  sake  of  uniformity,  and  to  conform  to  the  un- 
written laws  of  fitness,  it  is  often  necessary  in  bridge- 
designing  to  employ  metal  which  is  not  really  needed  for 
either  strength  or  rigidity. 

The  designer  who  recognizes  this  fact  will  usually  produce 
structures  of  liner  appearance  than  the  designer  who  ignores 
it  because  of  false  notions  of  economy, 


1 


18 


UK    I'ONTn4l.S. 


PniNcirLE  XII. 

Before  starting  a  design,  one  should  obtain  complete 
data  for  same. 

If  he  fails  to  do  so,  lie  will  genciiilly  liiive  to  niiikr  !ill(  iniioii 
after  ulteration  as  the  woik  progresses;  and,  as  one  cliange 
usuall}'  entails  several  others,  it  will  result  that,  by  the  lime 
the  work  is  tinished,  enough  labor  will  have  been  expended 
thereon  to  complete  two  such  designs,  for  which  proper  data 
were  furnished  at  the  outset. 

Principle  XIII. 

A  skew- bridge  is  a  structure  the  building  of  which  should 
always  be  avoided  when  it  is  practicable. 

It  is  generally  possible  to  stpiare  the  crossing  either  by 
swinging  the  centre  line,  or  by  lengthening  the  spans  and 
squaring  the  piers  or  abutments,  tionielinies,  however,  it  is 
not  practicable  to  do  either,  ir;  which  case  the  engineer  must 
make  tlie  best  of  a  bad  business.  The  objections  to  a  skew- 
bridge  are  these  ;  First,  it  is  fuliy  twice  as  troublesome  to  de- 
sign as  a  s(piare  structure;  second,  the  liability  to  error  in  both 
shop  and  lield  is  greatly  increased  by  the  skew  ;  and,  third, 
the  resulting  bridge  is  never  so  rigid,  nor  is  it  so  satisfactory 
iu  a  number  of  particulars,  as  a  bridge  without  this  objection- 
able feature. 

PniNciPii,  XIV. 

The  best  modern  practice  in  bridge  engineering  does  not 
countenraicc  the  building  of  structures  having  more  than  a 
single  system  of  cancellation,  except  in  lateral  systems 
where  the  resulting  ambiguity  of  stress  distribution  is  of 
minor  importance. 

Some  engineer  may  question  the  coirc  Itiess  of  this  asser- 
tion ;  but  if  1h'  will  glance  through  the  author's  paper  on 
"  Some  Disputed  Points  in  Uailway-jiiidge  Designing"  pub- 
lished in  the  February  and  Mirch,  180,.  numlier  of  tin;  Trdns- 
(irf/'nns  of  the  Anierjcivn  !^'n;iciy  of  ("ivil  IOiigiui'«rs,  he  wi'l  see 


.. 


i 


FIRST    riUKCirLES   OF    UKSIGNINU. 


19 


implete 

III. 'It;  on 
cliaiigo 
le  lime 
XTidod 
iv  data 


; 


lliiit,  as  a  wjiole,  the  engineering  profcssio.i  indorses  tlie  state, 
nieut.  The  only  ordinary  cases  wliere  multiple  systems  are 
employed  nowadays  are  those  of  the  lattice  girder  and  the 
Whipple  truss.  The  former  is  conceded  by  the  leading  bridge- 
designers  to  be  unscientific,  clumsy,  often  unsightly,  and 
always  uneconomical;  and  as  for  ihe  latter,  there  is  no  longer 
any  excuse  for  its  use,  because  it  lias  been  ousted  from  ihe 
jiosltion  k  used  to  hold  by  the  Petit  truss,  which  excels  it  in 
every  particular,  including  appearance,  economy  of  material, 
and  mathematical  correctness. 


Principi.e  XV. 

The  employment  of  a  redundant  member  in  a  truss  or 
girder  is  never  allowable  under  any  circumstances,  unless 
it  be  in  the  mid-panel  of  r-  span  having  an  odd  number  of 
panels,  in  which  case,  for  the  sake  of  appearance,  two  stiff 
diagonals  can  be  used. 

The  reason  for  this  is  ^'erfectly  clear  when  one  considers 
that  it  takes  extra  metal  to  I'uiid  the  said  redundant  member, 
aiil  that  its  use  upsets  the  calculations  of  stresses,  rendering 
them  in  fact  iusolvable.  A  lengthy  treatise  was  published  a 
few  years  ago  in  India  upon  a  method  of  finding  stresses  in 
redundant  members,  in  which  much  good  mental  energy  was 
wasted,  for  the  entire  book  might  have  been  written  in  these 
four  words:  "Never  use  such  members."  It  is  nut  often  that 
an  American  engineer  is  found  guilty  of  tniploying  unneces- 
sary pieces  in  his  designs,  but  one  cannot  say  the  same  of  his 
European  brethren. 

PniNCIPLK  XVI. 

The  use  of  a  curved  strut  or  tie  in  bridge-designing  for  the 
sake  of  appearance  (or  for  any  other  reason)  is  an  abomina- 
tion that  cannot  for  an  instant  be  tolerated  by  a  good  de- 
signer. 

It  is  hardly  necessary  to  make  such  a  forcible  remark  as  this 
to  American  engineers,  although  in  travelling  about  the  United 
Stales  one  occasionally  runs  across  a  violatjoa  of  the  self- 


20 


I)K    roNTIlU'S. 


evident  iinderlying  principle  involved  in  tliis  statement ;  but 
the  publislied  records  of  some  of  tlie  greatest  bridges  designed 
by  Englisli  engineers  sliow  the  use  of  pieces  of  trusses  so 
curved  that  actually  tliere  is  compression  on  one  extreme  fibre 
and  tension  on  the  other.  Archilecturnl  effect  is  undoubteily 
livery  commendable  feature  in  ])ridg(!-designing  ;  but  its  adop- 
tion is  no  excuse  for  the  violation  of  the  fundamental  principle 
that  every  compression  or  tension  member  of  a  truss  or  open- 
webbed  girder  should  be  absolutely  siraighl  from  end  to  end. 
Tt  seems  almost  unnecessary  to  state  that  the  appearance;  of 
curvature  can  be  obtained  i)y  employing  short  panels  and 
making  each  chord-length  straight  between  panel  points. 

PHINCII'LE    XVII. 

In  all  structural  metal-work,  excepting  only  the  machi- 
nery for  operating  movable  parts,  no  torsion  on  any  mem- 
ber should  be  allowed  if  it  can  possibly  be  avoided ; 
otherwise,  the  greatest  care  must  be  taken  to  provide 
ample  strength  and  rigidity  for  every  portion  of  the  struc- 
ture affected  by  such  torsion. 

It  is  not  often  that  this  question  arises;  nevertheless  it  is 
sometimes  forced  upon  the  consideration  of  llie  engineer.  It 
came  up  lately  in  the  author's  practice  in  the  case  where  an 
elevated-railroad  exit-stairway,  having  at  mid-height  a  landing 
and  a  180-degree  turn,  had  to  be  supported  by  a  single  column 
in  order  to  comply  with  the  demands  of  adjacent  property 
owners. 

PUIXCIPI.K   XVIII. 

The  gravity  axes  of  all  the  main  members  of  trusses  and 
lateral  systems  coming  together  at  any  apex  of  a  truss  or 
girder  should  intersect  in  a  point  whenever  such  an  ar- 
rangement is  practicable ;  otherwise  the  greatest  care  must 
be  employed  to  insure  that  all  the  induced  stresses  and 
bending  moments  caused  by  the  eccentricity  be  properly 
provided  for. 

This  is  an  important  rule  that  is  more  often  honored  in  the 
breach  than  in  the  observancaj  ;  in  fact,  as  far  as  the  author 


FIKST   PRINCIPLES  OF   DESIGNING. 


21 


;  but 
gned 
es  so 
fibre 
•felly 
idop- 
ciple 
opeii- 

C'licl. 

vo  of 
uiid 


knows,  tbere  are  only  n.  very  few  bridges  iu  which  the  desired 
intersection  iu  a  single  point  of  the  axes  of  all  niembors 
assembling  at  each  apex  is  accomplished  ;  and  iu  most  struc- 
tures where  eccentricity  exists  for  want  of  such  intersection, 
its  prejudicial  effects  are  upt  duly  recognized  and  provided 
for. 

Principle  XIX. 

Truss  members  and  portions  of  truss  members  should 
always  be  arranged  in  pairs  symmetrically  about  the  plane 
of  the  truss,  except  in  the  case  of  single  members,  the  axes 
of  which  lie  in  said  plane  of  truss. 

One  occasionally  sees  a  violation  of  this  principle  even  in 
important  bridges  ;  but  experience  with  structures  in  which 
it  was  iguorcd  has  l>een  such  as  to  show  most  clearly  that  this 
(;anuot  be  done  with  impunity,  for  the  torsion  resulting  from 
eccentrically  connected  adjustable  members  is  patent  even  to 
the  uninitiated. 

PUINCIPLK  XX. 

In  proportioning  main  members  of  bridges,  symmetry  of 
section  about  tw^o  principal  planes  at  right  angles  to  each 
other  is  a  desideratum  to  be  attained  whenever  practicable. 

Of  course  in  top-chord  nnd  inclined  end-post  sections, 
wldcli  should  l)e  designed  with  a  cover-plate,  symmetry  about 
both  principal  planes  is  not  attainable.  The  objectionable 
features  caused  by  want  of  it,  however,  are  provided  against 
by  the  next  axiom. 

Principle  XXI. 

In  both  tension  and  compression  members  the  centre 
line  of  applied  stress  must  invariably  coincide  with  the 
axial  right  line  passing  through  the  centres  of  gravity  of 
all  cross-sections  of  the  member  taken  at  right  angles 
thereto. 

Until  very  lately  this  iniiporlanl  i)riuciple  nas  been  simply 
ignored,  the  effect  being  that  the  allowed  intensities  of  work- 


22 


T>K    PONTTHUS. 


iiig  strt'sscs  are  often  excicded  l)y  from  fifty  to  one  huudred 
per  cent  because  of  the  eccentricity  thus  involved. 

Pk1N(  II'LE  XXII. 

The  principle  of  symmetry  in  designing  must  be  carried 
even  iuio  the  riveting ;  and  groups  of  rivets  must  be  made 
to  balance  about  central  lines  and  central  planes  to  as  great 
an  extent  as  is  practicable. 

The  viohition  of  tliis  i)rinciple  was  exceedingly  common 
not  very  long  ago  ;  and  even  to  day,  when  checking  the  shop 
drawings  (f  some  of  the  lending  bridge-manufacturing  compa- 
nies, the  author's  assistants  have  to  correct  occasional  depart- 
ures therefrom. 

PurNCII'LE  XXIII. 

In  proportioning  members  of  bridges  to  meet  stresses 
and  combinations  of  stresses  it  is  important  to  consider 
duly  the  quality,  frequency,  and  probability  of  the  action 
of  said  stresses  or  combinations  of  stresses. 

As  a  rule,  standard  specilications  lake  care  fairly  well  of 
this  subject;  nevertheless  there  will  often  occur  iu  one's 
practice  casts  which  they  do  iu)t  cover.  The  quality  of  stress 
should  be  considered  in  deternuning  the  sectional  area  of  the 
member,  because  the  greater  the  impact,  other  things  remain- 
ing the  same,  the  smaller  should  l;e  the  intensity  of  working 
stress.  Tiie  frecjuency  of  application  of  stress  should  lie 
consiiii'ied,  because,  if  a  certain  stress  or  combination  of 
stresses  be  of  frequent  occurrence,  a  small  intensity  of  work- 
ing stress  should  be  adopted,  while  for  very  iiifretpu-nt  occur- 
rences the  intensity  can  he  taken  considerably  higher. 

Finally,  the  probability  of  the  application  of  a  certain 
load  or  loads  should  be  considered  ;  because  for  inevitable 
loads  or  coml)iiiations  of  loads  the  mital  sliould  be  strained 
fairly  low,  while  for  highly  improbable  loads  or  combinations 
of  loads  it  is  legitinuite  to  strain  much  higher.  Just  hcie  the 
author  wishes  to  slate  most  clearly  and  emphatically,  that  to 
indorse  the  points  asserted  under  this  heading  one  need  not 


FIUST   r-UKVClPLKH   OF    DESKiNlNO. 


23 


I)(!  a  bolievnr  in  tlio  (ioctiines  of  Wolilcr  niid  Wcyrauch,  and 
ill  tiio  tiu'ory  of  the  fiitigiui  of  iii(;t;il.s,  because  one's  conunon 
sense  will  lead  him  to  jiroporlion  seclion.s  of  bridge  jneinl)er.s 
in  uceordance  with  tiie  foregoini,'  views. 

In  the  spicitie.'ilions  given  in  C'lnipteis  XIV.  and  XVI.  tlic 
inipiu;!  fi)rn\ulie  and  the  increased  int(  nsitie.s  for  eoinbiualioiis 
of  stresses  involving  lliose  due  to  wind  loads  take  care  of  this 
feature  of  design  for  all  structures  excepting  high  railroad 
trestles,  in  wliich  latter  tlie  designer's  professional  judguieut 
cannot  well  be  eliminated. 

PllINCII'I.K   XXIV. 

In  all  main  members  having  an  excess  of  section  above 
that  called  for  by  the  greatest  combination  of  stresses,  the 
entire  detailing  should  be  proportioned  to  correspond  with 
the  utmost  working  capacity  of  the  member,  and  not 
merely  for  the  greatest  total  stres.^  to  which  it  may  be 
subjected.  In  this  connection,  though,  the  reduced  capac- 
ity of  single  angles  connected  by  one  leg  only  must  not  be 
forgotten. 

It  is  almost  needless  to  state  that  most  engineers,  especially 
those  connected  with  contracting  companies,  will  disagree 
willi  theautiioron  the  correctness  of  tins  statement ;  neverthe- 
less he  has  yet  to  see  the  lirsl  case  where  adherence  to  the 
principle  would  involve  improper,  clumsy,  or  inappropriate 
construction.  If  it  be  right,  for  any  reason,  to  use  an  e.xlra 
amount  of  metal  in  t  lie  section  of  a  member,  why  is  it  not  also 
right  to  design  lh;il  mend)er  throughout  so  that,  if  tested  to 
destruction,  it  would  fail  as  a  whole  and  not  in  a  detail?  It 
.seems  to  the  autlior  that  the  considerations  which  require 
extra  section  would  demand  either  extra  strength  or  extra 
rigidity,  or  both,  in  the  details  as  well  as  in  tlie  section  itself. 

PiUNrii'i.K  XXV. 

In  every  bridge  and  trestle  adequate  provision  must  be 
made  for  the  contraction  and  expansion  of  the  metal. 

Neglecting  to  comply  with  this  principle  has  often  beeu  the 
cause  of  failure  and  disaster. 


24 


1)K    I'ONTIIU'S. 


Pkinciim.k  XXVI. 

No  matter  how  great  its  weight  may  be,  every  ordinary 
fixed  span  should  be  anchored  effectively  to  itH  aupports  at 
each  bearing  on  same. 

At  one  cud  it  sbould  be  ancliorctk  ininiuvat)jy,  and  (it  ibe 
other  so  as  to  providt;  for  loiii^itiidiniil  expansion  and  contrac- 
tion. Such  anciiorage  prevents  I  lie  dislodt;inj^  of  the  striic- 
tiire  by  \vind-p^e^^8ure  or  by  an  accidental  blow  from  a  mov- 
ing object. 

PuiNCIPhK   XXVII. 

The  bridge-designer  should  never  forget  that  it  is  essen- 
tial throughout  every  design  to  provide  adequate  clearance 
for  packing,  and  to  leave  ample  room  for  assembling  mem- 
bers in  confined  spaces. 

Tbere  is  no  more  fruitful  source  of  profanity  for  bridge- 
ereclors  than  the  neglect  of  this  principle;  and  as  nearly  every 
(le.>^igner  has  to  spend  a  year  or  two  in  learning  toallow  enough 
clearance,  it  follows  that  bridge-erec-tors  shonld  be  given  the 
benelil  of  "  extenuating  circumstances"  when  brought  to 
judgment  for  their  notorious  addiction  to  the  use  of  strong 
language. 

PiUNcrvLE  XXVIII. 

Although  for  various  reasons  engineers  are  agreed  that 
field-riveting  should  be  reduced  to  a  minimum,  such  an 
opinion  should  not  be  allowed  to  militate  against  the  em- 
ploymsnt  of  rigid  lateral  systems.  All  designs  should  be 
arranged  so  that  the  field-rivets  can  be  driven  readily. 

One  of  the  main  reasons  for  the  unsatisfactory  condition  of 
most  of  the  elevated  railroads  of  this  country  is  that  their  de- 
signers endeavored  in  every  possible  way  to  avoid  field-rivet- 
ing, so  as  to  keep  down  the  cost  of  erection,  and  in  so  doing 
fa'.led  to  develop  the  requisite  amount  of  rigidity  in  the  struc- 
tures. 


FIUST    I'KINM'II'LES   OF    DKSKJN'ING. 


Ji5 


PniNcu'LK  XXIX. 

Rivets  should  not  be  used  in  direct  tension. 

Ill  the  (lays  of  iron  rivt'ts  this  was  an  important  roqulrc- 
niont,  for  llic  louson  Ihut  llic  siiaiiks  were  often  hooverstrainetl 
in  I  ooiing  that  the  heads  won d  lly  oil ;  but  tliis  does  not  oc- 
cur witli  steel  rivets.  Nevertlieless  it  is  advisflblt;  loadliere  to 
the  rule,  except  for  very  unimportant  members  where  there  is 
a  great  excess  in  the  number  of  rivets  above  the  theoretical 
requirements 

"'  KINCIPLE  XXX. 

For  members  of  any  importance  two  rivets  do  not  m«ke 
an  adequate  connection. 

For  such  details  us  lattice  burs,  of  course  two  rivets  or  even 
one  rivet  at  each  cud  will  sutUce  ;  but  where  a  direct  calcula- 
ble stress  comes  on  the  piece,  and  only  two  rivets  at  each  end 
arc  used,  it  will  be  found  that  they  will  work  loose,  while,  if 
three  are  used,  they  will  not,  unless  they  be  overstrained  by 
the  caic'jl«t(id  stress  on  the  piece. 

Principi.k  XXXT. 

Designs  must  invariably  be  made  so  that  all  metal-work 
after  erection  shall  be  accessible  to  the  paint-brush,  except, 
of  course,  those  surfaces  which  are  in  close  contact  either 
with  each  other  or  with  the  masonry. 

This  clause  very  properly  cuts  out  the  use  of  closed  col- 
umns, which  are  a  fruitful  source  of  condemnation  of  old 
bridges. 

PUINCIPI,E   XXXII. 

In  multiple-track  structures,  if  any  bracing-frames  be 
used  between  panel  points  to  connect  the  longitudinal  gir- 
ders of  adjoining  tracks,  they  must  be  designed  without 
diagonals,  in  order  to  prevent  the  transference  of  any  ap- 
preciable portion  ef  the  live  load  from  one  pair  of  girders 
to  any  other  pair  of  same. 


2G 


I)E    POKTIHUS. 


Such  a  traiisfereuce  would  be  doubly  injurious  ;  because  it 
would  throw  on  some  of  the  girders  more  live  load  than  they 
were  proportioned  to  carr}',  and  at  the  same  time  it  would 
probably  overstrain  the  diagonals  and  their  connections,  and 
would  certainly  tend  to  distort  laterally  the  flange  angles  of 
the  longitudinal  girders. 

Principle  XXXIII. 

In  bridges,  trestles,  and  elevated  railroads,  the  thrust 
from  braked  trains  and  the  traction  should  be  carried  from 
the  stringers  or  longitudinal  girders  to  the  posts  or  col- 
umns writhout  producing  any  horizontal  bending  moment 
on  the  cross-girders. 

This  is  a  late  requirement  of  the  author's,  that  has  been  em- 
ployed in  ids  designs  for  a  few  years  past.  Its  correclULSs  was 
established  in  his  before  mentioned  paper  on  Elevated  Kail- 
roads. 

PlUNCIPLE   XXXIV. 

In  trestles  and  elevated  railroads  the  columns  should  be 
carried  up  to  the  tops  of  the  cross-girders  or  longitudinal 
girders  and  be  effectively  riveted  thereto. 

The  correctness  of  this  proposition  also  was  established  in 
the  said  paper  on  Elevated  Railroads. 


PiuNCirLB:  XXXV. 

Every  column  that  acts  as  a  beam  also  should  have  solid 
webs  at  right  angles  to  each  other,  as  no  .eliance  can  be 
placed  on  lacing  to  carry  a  transverse  load  down  the 
column. 

The  truth  of  this  propo.sitlon  is  evident  when  one  retiects 
that  a  single  loose  rivet  or  a  single  bent  lacing-bar  in  the  whole 
line  of  lacing  will  prevent  the  latter  from  carrying  as  a  web  a 
transverse  load.  Loose  rivets  and  bent  lacing-bars  iire,  unfor- 
tunately, not  uncommon  in  structural  metal-work. 


FIllST   PRINCrPLKS   OF    DESIGNING. 


2: 


le  it 
licy 

Ul(i 
llU(i 

of 


PUINOIPLK    XXXVI. 

In  trestles  and  elevated  railroads  every  column  should 
be  anchored  so  firmly  to  its  pedestal  that  failure  by  over- 
turning or  rupture  could  not  occur  in  the  neighborhood  of 
the  foot  if  the  bent  were  tested  to  destruction. 

As  long  ago  as  1891  the  author  designed  pedestals  which  in- 
volved truly  fixed  ends  for  colunui  feet;  l)ut  it  is  only  wilhiii 
tlie  last  three  years  tliat  such  a  detail  lu.s  begun  to  come  into 
general  use.  The  ordinary  connection  of  columns  lo  pedestals 
by  an  unchor-l)()lt  iit  each  of  the  four  corners  of  the  bedplate 
is  extremely  weak  and  ineirective. 

PUINCIPLK    XXXVII. 

All  pedestals  for  trestles,  viaducts,  and  elevated  railroads 
should  be  raised  to  such  an  elevation  as  to  prevent  the 
accumulation  of  dirt  and  moisture  about  the  column  feet, 
and  all  boxed  spaces  in  the  latter  should  be  filled  with 
extra-rich  Portland-cement  concrete. 

The  neglect  of  these  precautious  causes  the  rapid  deteriora- 
tion of  tlic  metal  at  bases  of  columns,  and  thus  shortens  the 
life  of  the  structure. 


PUINCII'LE   XXXVIII. 

In  designing  short  members  of  open  webbed,  riveted 
work,  it  is  better  to  increase  the  sectional  area  of  the  piece 
from  ten  to  twenty  five  per  cent  than  to  try  to  develop  the 
theoretical  strength  by  using  supplementary  angles  at  the 
ends  to  connect  to  the  plates. 

This  principle  is  based  upon  the  resvdts  of  some  late  tests  of 
th(!  author's  on  tlie  strengtli  of  single  angles  and  pairs  of  angles 
(•oiint'clod  l)y  one  leg  only,  by  which  lie  f "Uud  tliat  6"  X  ''a" 
angl(!s  thus  connected  dev('loi)ed  idnety  p<  cent  of  the  ulti- 
mate strenglli  of  a  flat  l)arof  eipuil  net  section,  and  that3"X3" 
angles  developed  seventy-five  per  cent  of  same. 


28 


DE   PONTIBUS. 


Pkinciple  XXXIX. 

Star-struts  formed  of  two  angles  with  occasional  short 
pieces  of  angle  or  plate  for  staying  same  do  not  make  satis- 
factory members.  Better  results  are  obtained  by  placing 
the  angles  in  the  form  of  a  T. 

Tlie  truth  of  tliis  slatemori  .".<  '-^ished  by  auolher  series 
of  e.vperlments  of  the  aulhor  .>  niuat  at  the  same  time  as  were 
the  liist-meutioned  tests.  The  specimen  cuhimiis  did  uot  de- 
velop on  the  average  more  than  seventy-five  per  cent  of  the 
ri'sistance  they  should  have  developed  according  to  tlie  usual 
straight-Hue  formula  lor  metal  of  the  same  teu&iile  strength. 

PUINCIPLE  XL. 

In  making  estimates  of  weights  of  metal  the  computer 
should  always  be  liberal  in  allowing  for  the  weight  of 
details. 

It  is  the  author's  experience  that,  in  nenrlv  every  case,  the 
weight  of  the  tinished  structure  exceeds  s;:i;i»tO  tiie  estimated 
weight,  and  mainly  on  account  of  the  o  i  lOre  metal  for 

details  than  was  figured  upon.  Ofcou;'.  i  >r  sets  out  de- 
liberately to  "skill  "  a  bridge  so  as  to  save  i  i.  tlitj  metal  he 
can,  the  actual  weights  of  details  may  be  made  to  underrun 
the  estimate;  but  such  a  practice  is  most  reprehensible. 


I 


Tl: 

the  ( 

«Fo 

poin 

ferr« 

strui 

by  ^ 

ticu 

che< 

whi 

agrj 

best 

B 

pult 
stru 


Principle  XLI. 

In  general  details  must  always  be  proportioned  to  resist 
every  direct  and  indirect  stress  tb,"'*  may  ever  come  upon 
them  under  any  possible  condition,  -v^fchout  subjecting  any 
portion  of  their  material  to  a  stresu  -^v  ^ter  than  the  legiti- 
mate corresponding  working  stress. 

This  principle,  w'li'  u  has  been  given  before  in  several  of 
the  authov  ^,  |;n;vior.  ■  works  on  bridges,  involves  the  whole 
theory  of  oridge  ue'i;iii..ig. 


FlliST   rUINCIPLES   OF    DESIGNING. 


'^9 


lort 
itia- 
ting 


Ties 
ere 
de- 
tbe 

sual 

h. 


Principle  XLII. 

There  is  but  one  correct  method  of  checking  thoroughly 
the  entire  detailing  of  a  finished  design  for  a  structure,  viz.: 
"  Follow  each  stress  given  on  the  stress  diagram  from  its 
point  of  application  on  one  main  member  until  it  is  trans- 
ferred completely  to  either  other  main  members  or  to  the  sub- 
structure, and  see  that  each  plate,  pin,  rivet,  or  other  detail 
by  which  it  travels  has  sufficient  strength  in  every  par- 
ticular to  resist  properly  the  stress  that  it  thus  carries; 
check  also  the  sizes  of  such  parts  as  stay-plates  and  lacing, 
which  are  not  affected  by  the  stresses  given  on  the  di- 
agram, e  d  see  that  said  sizes  are  in  conformity  with  the 
best  modern  practice." 

But  to  do  all  this  as  it  sliould  be  done  necessitates  the  coni- 
puter's  being,  in  tiie  highest  sense  of  the  term,  an  "expert  on 
slriiclural  metal- work." 


CHAPTER  III. 


TRUE  ECONOMY   IN   DESIGN. 


TnEATiSE  after  trcntiso  has  been  written  npon  the  subject 
of  economy  iu  superstructure  design,  but  unfortunately  the 
result  is  simply  a  waste  of  good  mental  energy ;  for  the 
writers  thereof  invariably  attack  the  problem  by  moans  of 
complicated  mathematical  investigations,  not  recognizing  the 
fact  that  the  questions  they  endeavor  to  solve  are  altogether 
too  intricate  to  be  undertaken  by  mathematics.  The  object 
of  each  investigation  ai)|iears  to  have  been  to  establish  an 
equation  for  the  economic  depth  of  truss,  or  that  depth  which 
corresponds  to  the  minimum  amount  of  metal  required  for 
said  truss;  and,  to  start  the  investigation,  it  seems  to  have 
been  customary  to  make  certain  assumptions  which  are  not 
even  approximately  correct.  For  instance,  the  principal  as- 
sumption of  several  treatises  in  French  and  English  is  that 
the  .sectional  area  and  the  weight  of  each  member  of  a  truss 
are  directly  proportional  to  its  greatest  stress;  or,  in  other 
words,  that  in  proportioning  all  members  of  trusses  a  constant 
intensity  of  working  stress  is  to  be  used,  while  in  reality  for 
modern  steel  bridges  the  intensities  vary  from,  .say,  6000 
pounds  up  to  15,000  pounds,  or,  when  impact  is  provided  for, 
up  to  18,000  pounds,  and  when  both  impact  and  wind  stresses 
are  included,  up  to  nearly  34,000  pounds.  Again,  no  distinc- 
tion is  made  between  tension  and  compression  members,  and 
no  account  is  taken  of  the  greatly  varying  amounts  of  their 
percentages  of  weights  of  details. 

There  is,  however,  oik;  mathematical  investigation  concern- 
ing economic  truss  depths  which,  in  the  author's  opinion,  is 
approximately  correct,  and  which  is  based  on  assumptions 

30 


THUE    Kt'OJSOMY    IX    DESIGN. 


U 


tliat  iiro  very  nearly  true  •  but  it  liolds  good  only  for  parallel 
chords.     It  is  this  : 

Let  A  =  weight  of  the  chords, 
Ji  —  weight  of  ihc  web, 
(J  ~  weight  of  the  truss, 
and     I)  =  depth  of  truss. 


Then 


yl  -^  /?. 


But  the  weight  of  the  chords  varies  inversely  as  the  depth, 

(I 
or  A  =  jr,  and  the  weight  of  the  web  varies  ilireclly  as  the 

depth,  or  B=  bl),  where  a  and  b  are  constants  ;  and  therefore 


a 


C=-^  +  bD. 


If  O  is  to  be  made  a  minimum,  we  shall  have,  by  dilleren- 
tiation, 

(f£  _  _  ^i 
dl)  "       D 


,  +  i  =  0. 


or 


4        Ji 

-  ~  +  ^  =  0.     or     A  =  B. 


As  the  second  differential  coetlicient,  after  substitution 
according  to  tiie  usual  method  for  maxima  and  minima, 
conies  out  ]>osilive,  the;  rcMilt  obtained  corresponds  to  a 
inininuiui. 

From  this  it  is  evident  that,  for  trusses  wiiii  parallel  chords, 
the  greatest  economy  of  material  will  jjrevail  wlie.i  the  weight 
of  the  chords  is  equal  to  the  weight  of  the  web.  The  author 
has  vcrilied  this  lonclusion  by  checkiuL^  tin;  weights  of  chords 
and  webs  in  a  number  of  finished  designs,  limliug  it  to  be 
absolutely  reliable.  However,  it  is  not  of  much  practical 
value,  because  the  economic  depths  of  trusses  with  piiralle' 
chords  are  pretty  well  known  ;  and,  ag!un,  when  spans  are  in 
excess  of  175  or  200  feet,  the  chords  of  through-bridges  are 
seldom  nuide  parallel.     Moreover,  the  best  depth  to  u^e  is  not 


32 


I)K    rONTIBUS. 


often  the  one  which  gives  the  least  weight  of  metal  in  the 
trusses. 

Tlie  iiuthor  finds  by  experience  tluU,  for  trusses  with  po- 
lygonal to[)  chords,  the  erononiio  depths,  as  fur  as  weiglit  of 
metal  is  concerned,  are  generally  much  greater  than  certain 
iujportant  condit'ons  will  permit  to  be  used.  For  instance, 
especially  in  single-track  bridges,  after  a  certain  truss  depth 
is  exceeded,  tiie  overturning  effect  of  the  wind-pressure  is  so 
great  as  to  reduce  the  dead-load  tension  on  the  windward  bot- 
tom chord  to  such  an  extent  that  the  compression  from  the 
wind  load  carried  by  the  lower  lateral  system  causes  reversion 
of  stress,  and  such  reversiou  eye-bars  are  not  adapted  to  with- 
stand. A  very  deep  tru.ss  recpiires  an  expensive  traveller,  and 
to  decrease  the  theoretically  economic  dei)th  increases  the 
weight  but  slightly  ;  hence  it  is  really  economical  to  reduce 
the  deptli  of  both  truss  and  trav(;ller. 

Again,  the  total  cost  of  a  structure  does  not  vary  directly  as 
the  total  weight  of  metal,  for  the  reason  that  an  increase  in 
the  sectional  area  of  a  piece  adds  nothing  to  the  cost  of  its 
manufacture,  and  but  little  to  the  cost  of  erection  ;  so  it  is 
only  for  raw  material  and  freight  that  the  expense  is  really 
increased.  Hence  it  is  generally  best  to  use  truss  depths  con- 
siderably less  than  those  which  would  re([uire  the  minimum 
amoimt  of  metal.  For  parallel  chords,  tlie  theoretically  eco- 
nomic truss  depths  vary  from  one  fiflli  of  the  span  for  spans 
of  100  feet  to  ai)0ut  one  sixth  of  tlie  span  for  spans  of  '300  feet; 
but  for  modern  railway  through-bridges  tlie  lea.st  allowable 
truss  depth  is  about  28  teet,  unless  suspended  tloor-beams  be 
used,  a  detail  whi(;h  very  properly  has  gone  out  of  fashion. 

In  two  five-hundred -foot  spans  of  a  combined  railway  and 
highway  bridge  the  aullior  employed  a  truss  depth  of  seventy- 
two  feet;  but  this  was  determined  by  the  reversal  of  stress  in 
bottom  chords  through  wind-pressure.  A  greater  depth,  if 
permissible,  would  have  caused  a  saving  in  total  weight  of 
metal. 

In  a  design  of  the  author's  for  a  five-hundred-and-sixty- 
foot  span  a  truss  depth  of  ninety  feet  was  adopted,  but  in 
this  case   the   live   load   was   very   great,  varying   from  tea 


TRUE    ECONOMY    IN    DESIGN. 


33 


thousand  pounds  per  lineal  foot  for  short  spjins  to  eight 
thousand  pounds  per  lineal  foot  for  long  spans;  and  the  bridge 
is  twenty  per  cent  wider  than  in  the  case  of  the  two  five- 
hundied-foot  spans  just  mentioned. 

The  greater  the  live  loud  and  the  wider  the  bridge,  the 
greater  can  the  truss  depth  be  made  advantageously. 

The  little  mathimalical  investigation  given  in  this  chapter 
can  be  applied  with  advantage  to  plate-girder  bridges  and  to 
the  floor  systems  of  truss  bridges.  If  for  ordinary  cases,  in 
designing  plate  girders,  one  will  adopt  s\jcli  a  depth  as  will 
make  the  total  weight  of  the  web  with  its  splice-plates  and 
stilfening  angles  about  equal  to  the  weight  of  the  flanges,  he 
will  obtain  an  econoniicaily  designed  girder,  and  a  deep  and 
stiff  one.  For  long  spans,  iiowever,  this  arrangement  would 
make  the  girders  so  deep  as  to  become  clumsy  and  expensive 
to  handle;  consequently  when  a  span  exceeds,  say,  forty  feet, 
the  amount  of  metal  in  the  flanges  should  be  a  little  greater 
than  that  in  the  web;  and  the  more  the  span  exceeds  forty 
feet  the  greater  should  be  the  relative  amount  of  metal  in  the 
flanges. 

Concerning  economic  panel  lengths,  it  is  safe  to  make  the 
following  statement:  "Within  the  limit  set  by  good  judgment 
and  one's  inherent  sense  of  titness,  the  longer  the  panel  the 
greater  the  economy  of  materia!  in  the  superstructure."  Of 
course,  when  one  goes  to  such  an  extent  as  to  use  a  thirty-fool 
panel  in  an  ordinary  single-track  bridge  he  exceeds  the 
limits  referred  to,  because  the  lateral  diagonals  become  too 
long,  and  their  inclination  to  the  chords  becomes  too  flat  for 
rigidity.  Again,  an  extremely  long  panel  would  often  cause 
the  truss  diagonals  to  have  an  unsightly  appearance,  because 
of  their  snuill  inclination  to  the  horizontal. 

There  Is  another  mathematical  investigation  which  is  of 
practical  value.  It  relates  to  the  economic  lengths  of  spans, 
and  was  first  demonstrated,  in  print,  by  the  author  some  six 
years  ago  in  Indian  Engineering,  although  the  principle  was 
announced  three  years  before  then  in  th';  first  edition  of 
his  General  Specifications  for  Highway  Bridges  of  Iron  and 
Steel.     Strange  to  say,  many  engineers  failed  to  see  that  there 


34 


DE    I'ONTIHUS. 


is  any  diilerciicc  between  this  principle  and  an  old  practice  of 
forty  years'  standing.  The  principle  is  that  "  for  any  cross 
ing  the  greatest  economy  will  be  attained  wheu  the  cost  per 
lineal  foot  of  the  substructure  is  ecjual  to  the  cost  per  lineal 
foot  of  the  trusses  and  lateral  systems."  The  old  practice  was 
to  inake  for  economy  the  cost  of  a  pier  equal  to  the  cost  of  the 
span  that  it  supports,  or,  more  proi)erly,  equal  to  cue  half  of 
the  cost  of  the  two  spans  that  it  helps  to  support. 

Is  not  the  difference  between  these  two  methods  perfectly 
plain  ?  In  one  the  cost  of  the  pier  is  made  etpiid  to  the  cost  of 
the  trusses  and  laterals,  and  in  the  other  it  is  made  etpial  to  the 
cost  of  the  trusses,  laterals,  and  the  floor  system.  When  one 
considers  that  the  cost  of  the  floor  system  is  sometimes  almost 
as  great  as  one  half  of  the  total  cost  of  the  super's* ructuic,  lie 
will  recognize  how  faidty  the  old  nietliod  was. 

The  following  is  the  demonstration  of  the  principle,  sim- 
plified to  the  greatest  practicable  extent.  Let  us  assume  a 
crossing  of  indefinite  length,  for  which  the  depth  of  bed-rock 
is  constant,  and  let 

S  =  cost  per  lineal  foot  of  the  substructure, 
T  =  cost  per  lineal  foot  of  the  trusses  and  laterals 
F  =  cost  per  lineal  foot  of  the  floor  system, 
B  =  cost  per  lineal  foot  of  the  entire  bridge, 
L  =  length  of  span; 


and 
then 


B  =  S+  1'+  F. 


Now  if  we  assume  that  slight  changes  in  length  of  span  will 
not  affect  materially  the  sizes  of  the  piers,  the  cost  per  foot  of 
the  substructure  will  vary  inversely  as  the  span  length, 


or 


-S  = 


Again,  the  cost  per  foot  of  the  trusses  and  laterals,  for 
slight  chaniics  in  length  of  span,  may  l)e  assumed  to  vary 
nearly  directly  as  the  span  length;  hence  we  may  write  the 
equatiou 

T=  iL. 


TKUE    ECONOMY    IN    DESIGN. 


36 


le  of 

loss 
per 

liu-al 

I  was 
the 

If  of 


The  cost  per  foot  of  the  tloor  system  is  practically  inde- 
pendent of  the  span  length,  being  ii  function  of  the  panel 
length,  which  does  not  change  materially  with  the  span.  We 
now  have  the  eciualiou 


B  =  j^  +  tL  +  F, 

In  which  B  is  to  be  made  a  minimum. 
Dlllereutiating,  we  have  (as  F  is  a  constant) 

A  further  differentiation  shows  that  the  result  corresponds 
to  a  minimum. 

Ill  reality  tho  truss  weight  per  foot  increases  more  rapidly 
than  the  span  length.  If  r  is  the  ratio  of  span  lengths,  the 
truss  weights,  for  small  changes  in  span  lengths,  will  vary 
almost  exactly  according  to  the  ratio  ?•'  =  \(r  +  ?•').  On  the 
other  hand,  the  weight  per  foot  for  the  lateral  system  does 
not  increase  as  rapidly  as  the  span,  unless  the  per]  .id'cular 
distance  between  central  planes  of  trus.ses  also  inci eases. 
Unfortunately,  though,  the  gain  in  truss  weight  over  that 
given  by  the  assumed  theory  of  variation  is  generally  greater 
than  the  corresponding  loss  for  the  weight  of  lateral  system, 
consecpienily  the  combined  weights  per  foot  of  trusses  and 
laterals  generally  increase  a  tritie  faster  than  the  span  lengtii. 
This  is  i)artially  olfset  by  the  fact  that  the  pound  price  of 
metal  erected  and  painted  will  reduce  a  trifle  as  the  weight 
per  foot  increases. 

Again,  there  is  sometimes  a  small  error  in  the  assumption 
that  the  cost  of  the  piers  varies  inversely  as  the  span  length 
because  the  size  of  each  pier  may  have  to  be  increased  a  little 
to  accommodate  the  heavier  spans.  If  the  perpendicular  dis 
lance  between  central  planes  of  trusses  is  increased  because  o 
the  greater  span  length,  the  cost  of  each  pier  will  be  increase( 
because  of  its  greater  length;  but  this  will  occur  only  occa 
sionally. 


36 


HE    ro  XT  I  BUS. 


Ignoring  tlio  lalttir  coiillngiMicy,  tlie  two  errors  indicated, 
notwilhslnndiiig  the  fuel  timt  their  effects  :ne  juhlilive,  are  so 
small  us  not  to  iiirect  luateri.illy  tlu;  corri'(;tiiess  of  the  results 
of  this  invesiigation  concerning  economic  span  lenglhs. 

Thisdeiuonstratioii  proves  tliiit,  in  any  lavoiil  of  spans,  with 
the  conditions  assumed,  the  greatest  economy  will  hv.  attained 
when  the  cost  of  the  substructure  per  lineal  foot  of  biidgc;  is 
equal  to  the  cost  per  lineal  foot  of  the  trusses  and  lateral 
systems  Of  course  no  such  condition  as  a  bridge  of  in- 
definite extent  ever  exists,  nor  is  the  bed-rock  often  level  over 
the  whole  crossing  ;  nevertheless  the  principle  c;in  he  applied 
to  each  |)ier  and  the  si)ans  that  il  helps  to  s\ipport  l)y  making 
the  cost  of  each  pier  equal  to  one  half  of  the  total  cost  of  the 
trusses  and  Ipterals  of  both  spans.  Since  working  out  this 
demonstration  some  ten  years  ago,  the  author  has  made  a 
practice  of  checking  the  correctness  of  the  principle  thereby 
established,  by  comparing  the  cost  of  substructure  and  nuper 
structure  in  a  numoer  of  bridges  which  he  has  designed  and 
built,  with  the  result  that  he  finds  it  to  be  exact. 

The  principle  will  apply  also  to  trestles  and  elevated  roads, 
for  in  the  latter,  if  we  uuike  the  cost  of  the  stringers  or 
longitudinal  girders  of  one  span  equal  to  the  cost  of  the  bent 
at  one  end  of  same,  including  its  pedestals,  we  shall  obtain  the 
most  economic  layout.  In  an  ordinary  railroad  trestle,  con- 
sisting of  alternate  spans  and  towers,  il  will  be  necessary  for 
greatest  economy  t(>  have  the  cost  of  all  the  girders  in  two 
^-pans  (one  span  being  over  the  tow^er)  plus  tiie  cost  of  the 
longitudinal  bracing  of  one  tower,  equal  to  the  cost  of  the  two 
bents  of  said  tower,  including  their  pedestals. 

On  page  235  of  the  first  edition  of  Prof.  J.  R.  Johnson's 
"  Tireory  and  Practice  of  Modern  Fr:viicd  S:riu.'tures,''  Mr 
Jkyun  u.oes  this  method  of  the  auth'  ;      in  a  slightly  different 
form  for  determiinng  the  most  ecoiu)mi(;  ninuber  of  spans  to 
adopt  at  any  crossing,  establishing  the  equation, 


y  =  A-\-B-\-(x 


l)C+l^^p, 


TUIK    K(X)N().MY    IN    Id'.SKJK, 


a? 


iu  which  y  (the  total  cost  uf  bridge)  is  a  initiiiuum  when 


G, 


where  A  =  cost  of  two  end  ubutnu'iits  in  dollars  ; 

B  =  cost  of  tlie  floor  luid  tlmt  part  of  tiie  metal  weight 
whicli  reiimiiis  constant,  in  dollars; 

C  —  cost  of  out!  pitT  in  dollars,  assumed  as  constant ; 

I  —  lenj^tli  of  bridge  in  feet ; 

X  =  number  of  spans  ; 

p  =  price  of  nietiil  per  potiiid,  in  dollara; 

y  =  total  cost  of  Itridge  in  dollara  ; 

a  =  weiglit  per  foot  of  a  span  b  feet  in  length, 
riuis  fur  all  right  ;  but  then  he  makes  an  assumption  which 
will  not  be  correct  except  for  one  live  load,  for  one  set  of 
specific!'.' inns  and  for  sincle-track  railway  bridges,  viz.,  that 
for  piu-conuectt'l  spans 


aiK 


1 


a 


On  iiccoint  of  lliis  assum|)tioii  his  subsequent  table  of 
economic  span  lengths  is  not  in  any  sense  general,  but  is  true 
only  for  single-track  bridges  designed  for  one  standard  live 
load  and  according  to  one  standard  set  of  specifications  ;  while 
his  equations  ii-dd  good  for  bi  'ges  of  any  kind  .and  loading, 
including  higliway  as  well  as  railway  structures. 

As  a  check  on  the  correctness  of  Mr.  Bryan's  assumption 

that      =r  ^  for  siugle-track  bridges,  the  author  lias  looked  up 
a      k) 

some  of  his  designs  and  has  found  the  following  : 
For  a  375  ft.  through-span,  Class  X,  -  =  ^  ^- ;  for  a  362-ft. 

double-track  through-span,  Class  Z,  -  =  q  ,7  ;  »iiid  for  asimilur 

490-ft.  span,  -—  -— .     For  a  *J80-ft.  double-track  deck-span, 

a     y.o 


1^8 


DK    I'ONTHU'M. 


•  liissY,       =    —.1111(1  f(ir  II  Hiinilar  200.ft.  <leck-8pan  —  =  -    .. 
a      1).  1  «      U^i 

For  a  siiigk'-trju'k  2()0-ft.  tbrougli-spaii,  designed  by  a  con- 

triictiiig  liridge  coinpaiiy  and  checked  by  the  author,  "  =  jt%- 

The  detailing  thereof,  however,  was  ultiaeconomicai.  It  i^ 
but  fair  to  state  that  tlie  375-ft.  spun  is  about  two  feet  wider 
than  the  ordinary  single-track  bridges  of  such  span  lengths, 
wliich  causes  the  denominator  of  the  fraction  to  increase 
soniL'wliat.     It    is  evident,    tluv    -h,   that   the  assumptiou   of 


any  fixed  value  for-  is  unwai 


',  because  the  weights  per 


foot  of  trusses  and  laterals  for  spans  of  Classes  Z  and  U  of  the 
Coinproniise  Staiulard  System  will  vary  by  from,  say,  153  to  40 
per  cent,  according  to  the  span  length;  consequently  the  vahies 

of  -  would  vary  likewise. 

In  caNCs  of  sliucturcs  for  crossings  wliere  there  is  d.nigcr 
from  waslidul,  it  may  l)e  truly  (■{•oiiomical  to  use  metal  un- 
sparingly in  the  design,  in  onler  to  ensure  the  metal-work 
going  together  readily  and  willi  the  least  possible  delay  ;  and 
in  extreme  cases  it  wouiil  be  eminently  economical  to  adopt  a 
cantilever  design,  and  thus  rciduce  tlie  risk  of  washout  to  a 
minimum  l)y  the  expenditure  of  a  considerable  amount  of 
extra  nu'tal  for  the  su|)erslructure. 

'I'here  is  another  economic  feature  of  design,  which,  un- 
fortunately, has  been  overh)oked  continually,  viz.,  that  the 
most  economic  structure  is  the  one  for  which  the  first  cost,  phis 
th(!  capitalized  cost  of  annual  deterioration  and  repairs,  is  a 
minimum.  A  proper  consideration  of  this  economic  feature 
Would  cause  the  use  of  better  details,  hirger  sections  of  main 
members,  more  efticient  and  rigid  sway-bracing,  and  a  greater 
minimum  thickness  of  metal. 


CHAPTER   IV. 


ESTHETICS  IN   UESIGN. 


That  the  nietul  bridges  built  in  tlie  United  Stales  during  the 
lust  two  or  three  iecades  arc,  with  rare  exceptions,  anything 
but  models  of  excellence  in  respect  to  tlie  principles  of  restliet- 
ics,  no  eiigiiu'cr  is  likely  to  deny.  For  this  the  principal  rea- 
sons are  as  follows  : 

First.  Very  few  technical  schools  in  this  country  instruct 
their  engineering  students  at  all  in  architecture  :  and  not  one 
of  them  gives  to  that  important  branch  of  constructive  engi- 
neering tlie  attention  it  merits. 

Second.  As  most  American  enterprises  are  consummated 
with  a  small  amount  of  money  compared  with  what  might  be 
spent  advantagi'ousl}'  in  their  materialization  and  completion, 
there  are  sclilcm  any  f\inds  to  employ  in  decorating  tiie  work. 

Third.  American  engineers,  aa  a  rule,  appear  to  regard 
with  more  or  Ic m  contempt  oil  efforts  to  ingraft  aichitectnral 
ideas  ui)()n  engine,  ring  construction.  While  the  (ingi'^eering 
profession  is  <iidy  too  ready  to  criticise  architectural  construc- 
tion because  of  its  .uimerous  violations  of  the  principles  of  eu- 
gineering  pnutice,  it  docs  not  appear  to  see  thiit  the  converse 
of  tlie  proposition  holds  good,  viz.,  that  the  architectural  pro- 
fession hax  gooil  rea-son  to  (;riticise  severely  engineering  con- 
struction in  general  because  of  its  numerous  and  glaring  vio- 
lations of  the  principles  of  architecture.  Moreover,  in  no 
liranch  of  engimering  are  .^uch  violations  so  conunon  and  so 
pronounced  us  in  lluit  of  bridge  building. 

Fourth.  But  the  chief  factor,  the  one  which  has  had  more 
bad  intluence  than  all  the  others  combined,  is  the  custom  of 
letting  bridges  upon  competitive  designs  and  awarding  the 
coutract  to  the  lowest  bidder. 

39 


40 


DK    PONTlBt^S. 


For  miiny  years  prominent  iircliitects  Imve  very  justly  in- 
veli^hed  iigiiinst  the  inherent  ufjliness  of  Aineriejin  bridges.  In 
order,  therefore,  to  see  what  siicii  violiilions  of  aesthetics  in 
bridge-design inij  really  are,  and  to  what  extent  they  ean  be 
avoided,  the  author  has  asked  his  friend,  Henry  Van  I?runt, 
Es(i.,  of  the  architectural  firm  of  Van  Brunt  &  Howe,  who  is 
acknowledged  by  the  leading  members  of  iiis  profession  to  be 
one  of  tiie  foremost  living  musters  of  the  science  of  architec- 
ture, to  write  for  publication  in  this  treatise  a  letter  formulat- 
ing the  charges  of  himself  and  his  professional  brethren  against 
the  bridge-builders  of  this  country  in  respect  to  their  alleged 
offences  against  the  leslhetics  of  construction.  In  response  to 
this  request  Mr.  Van  Brunt  has  written  the  following  letter  : 

My  DEAR  Mr.  Waddell  : 

After  loolcin^  over  a  pi^rlioii  of  your  instructive  treatis.  on  bridKes, 
I  find  it  quite  iiupossll)le  to  comply  with  your  request  to  furnish  you 
with  prncticdl  sugt^estions  from  an  areiiitectunil  point  of  view  as  to 
grace  and  lieauty  of  design  in  .such  structures.  As  tliese  qualities  must 
be  developed  from  tiie  structure  itself,  as  they  must  be  evolved  from  its 
inherent  economical  and  practical  conditions,  and  as  they  cannot  be  suc- 
cessfully applied  to  it  as  iiii  afterthought,  it  woidd  be  unbecoming  for 
a.)y  layman  to  attetipt  to  show  by  what  process  this  evolution  is  to  l)e 
accomplished.  The  problem  is  not  an  easy  one  ;  it  is  not  to  be  solved  by 
theory,  or  by  any  accident  of  invention  or  ingenuity.  .\t  present,  at 
least,  it  can  oidy  be  treated  on  general  lines.  Indeed  there  is  no  one  liv- 
Ing,  I  fear,  who  can  suggest  a  specific  and  easily  applied  remedy  for  that 
disease  of  engineering  which  in  e.vpressed  in  the  curious  fact  that  the 
most  perfect  results  of  science,  at  least  in  the  art  of  steel-bridge  building 
as  now  understood  and  inculcated,  do  not  recognize  any  theory  of  l)eauty 
in  line  or  mass. 

It  is  the  business  of  the  architect  to  express  structure  an<l  jtnrpost! 
with  beauty.  It  is  the  l)usiness  of  the  engineer,  as  I  understand  it,  to 
make  structures  strong,  durable,  rigid,  and  economical:  to  apply  pure 
science,  excluding,  as  a  matter  of  princiide,  any  device  of  art  which, 
for  the  .sake  of  mere  ornamentation,  may  a<ld  to  his  fabric  a  i)ound  of 
unnecessary  weight  or  a  dollar  of  unnecessary  cost. 

It  caimot  be  denied  that  to  whatever  extent  the  exercise  of  this  prin- 
ciple may  have  affected  the  practice  of  engineers,  they  liave  succeeded, 
especially  as  '"egarda  bridge-building,  in  developing  a  structure  which 
Is  ill  every  essential  respect  orderly,  consistent,  and  i)rogresslve  fioni  a 
practical  poit'f  of  view.  From  year  to  year  this  development  towards 
nu'chanical  perfection  has  been  plainly  visible.  The  structure  of  fen 
years  ago  has  been  reasonably  and  properly  superseded  by  another  and 
better  structure,  indicating  a  process  of  growth  wlthotit  a  sliadow  of 


AESTHETICS    IX   DESIGN 


41 


caprice  ;  in  tliis  process  discovery  and  invention  have  had  their  proper 
influence,  uninterrupted  l)y  any  conservative  prejudice  or  by  any  theory 
of  deslfjfii  which  does  not  rest  ilireclly  on  j>ractical  cfmsiderations.  Unl, 
as  I  have  already  ol)served,  tliis  admiralde  and  prolific  proKi'ess  lias  not 
cari'ied  Willi  it  a  correspomlinx  pro>;ress  in  graci'  and  lieanty  "f  design. 
In  fact. ''lese  (pialities  seem  to  appear  in  an  invcrsi'  [)roportion  to  the 
development  of  the  structural  scheme  towards  the  jiractical  idea  of 
strength,  stability,  and  e<M)noiriy.  ("onse(iMently  the  stronjjer,  the  more 
rifjid,  the  more  economical  the  structure,  the  more  uncompromising  and 
the  more  hopeless  it  seems  to  he  in  respect  to  beauty.  The  modem 
steel-girder  or  cantilever  bridj^e,  while,  according  to  our  present  knowl- 
edge, it  Is  perfectly  adapted  to  its  uses  and  f\mctions,  is  in  nearly  every 
(;ase  an  offence  to  the  landscap<^  'ji  which  it  occurs.  Its  lines,  sjn<;e  they 
have  ceased  to  he  structural  curves,  have  become  liard  and  ascetic 
mathematical  expressions,  and  have  not  been  brought  into  any  sytn- 
pathy  whatever  with  the  natural  lines  of  the  stream  which  it  crosses, 
of  the  <)p|)osite  banks  which  it  eomiects.  of  the  mea<lows.  forests,  and 
mountains  among  which  it  is  placed.  All  sylvan  effects  of  harmotiy  are 
shocked  by  its  discordant  intrusinn.  The  vast  atjuedui^ts  of  the  Komans, 
the  arched  bridges  of  stone,  the  catenary  ciu'ves  of  the  modern  suspen- 
sion bridges  with  their  high  towers,  and  some  forms  of  bridges  con- 
structed with  bowstring  girders,  are  more  or  less  affiliated  with  the 
natural  <'oiiditi()n<,  so  that  they  give  no  shock,  save  frecpiently  of  pleas- 
ure at  their  expression  of  grace  and  fitness.  Hut  we  are  assured  that 
these  structural  forms  are  obsolete  or  are  beconung  obsolete,  and  that 
the  straight  Ijridge-truss  spamiing  from  i)ier  to  pier,  the  cantilever  over- 
hanging the  perilous  aliyss,  the  pivoted  draw  -sp.an,  all  constructed  with 
cold  geometrical  precision,  with  haril  unfeeling  lines  of  tension  and 
compression,  have  t.ikeii  their  place,  to  the  great  advantage  of  the  rail- 
roads and  the  greater  security  of  the  puldic.  It  is  'n  vain  that  the  con- 
scientious engineer  occasionally  attempts  to  compromise  with  grace  by 
ornaiuenting  his  inter.sections  by  rosettes  or  buttons  of  cast  iron,  or  by 
rearing  a  sort  of  'ch  or  portal  of  triumph  at  the  entrance  to  his  liridge 
with  a  lavish  di  ,play  of  metal  shell-work,  scrolls  of  forged  irou,  and 
tables  cast  ai  d  gilded  with  names  and  dates.  Hut  the  comi)romise 
comes  too  laie  ;  the  main  essential  lines  camiot  be  condoned  bj-  after- 
thoughts of  this  suit ;  and  as  far  as  the  eye  c.'in  see,  these  lines,  though 
they  may  satisfy  the  reason,  generally  atfront  the  sense  of  beauty. 

Now  it  seems  to  me  important  to  note  that  the  methods  of  nature 
always  culminale  in  infinite expressi(Mis  of  beauty,  an<l  that  beauty  is  an 
esstMitial  part  of  the  principles  of  natural  growth.  The  (Jreat  Creator 
never  makes  anything,  aniinate  or  inanimate,  ugly  m  making  it  stro;ig 
or  swift  or  durable,  or  in  fitting  it  to  the  economy  of  nature.  (Irace  id 
a  i>art  of  the  system  of  creation.  Is  It  reserved  for  man  in  his  secondary 
creation  to  make  things  luilovely  iti  proportion  to  th<Mr  complete  and 
perfect  adaptation  to  the  satisfaction  of  his  praclicral  needs!'  Is  this 
difference  signifl     nt  of  some  <|uality  which  is  wanting  In  our  science  ? 


1)K    I'ONTIBUS. 


But,  it  may  be  said,  if  a  steel-tnissed  bridge,  pcouoniically  and  wisely 
constructed  according  to  our  [irosent  liglit,  offends  our  ideals  of  grace 
and  beauty,  tlie  fault  perliaps  is  not  in  tiie  structure,  but  in  tlie  rigidity 
and  inunol>ility  of  the  ideals  wliicli  have  been  estahlisiied  by  conditiouH 
long  since  outgrown  in  the  i>rogi-ess  of  science.  The  attempts  of  the 
Ei\glish  bridge-builders  in  iron  in  the  early  i)art  of  the  century  to  meet 
thest*  old  ideas  resulted  in  constructions  which,  though  they  may  satisfy 
the  eye  of  the  artist,  and  combine  more  or  less  graeefullv  with  the  laud 
scape,  are  uneconomical  and  unscientific.  The  principles  of  structure 
involved  are  incorrect,  and  unnecessary  expense  was  incurred  in  forcing 
into  the  dtisign  features  conventionally  acceptal)le,  but  which  liad  notli- 
ing  to  do  witli  the  struv'tnre.  und  which  in  fact  were  a  hindrance  to  it, 
concealing  rather  than  illustrating  it. 

The  architect  will  not  find  it  diUflcultto  agree  with  Ids  broiher  tlie  en- 
gineer, that  a  mask  of  ornamental  cast  iron,  covering  lliwexsential  feat- 
nres  ''f  tlie  structure  in  order  to  for(,'e  upon  it  an  efi.  't  of  grace,  is 
illogical  in  the  extreme.  Indeeil,  a  great  modern  master  of  architecture 
has  laid  down  the  a.xi<>m:  "  A  foi'in  which  admits  of  no  explanation,  or 
which  is  mere  caprice,  cannot  be  beautifid;  and  in  architecture,  cer- 
tainly, every  form  which  is  not  inspired  by  the  structure  ought  tliere- 
fore  to  bi^  rejeeteil."    Tlie  <'onscieniious  modern  architect  aims  to  shape 

his  design   a rding   to  this   reasotialile   limitalioo,  and  be   has   been 

thereby  enabled  to  proiluce  occasioiuil  etTects  of  beauty  without  impos- 
ing on  his  composition  a  single  idea  which  is  not  suggested  eitlier  by  the 
structure  or  by  the  use  of  th<- buildiii'..'.  Kvimi  a  factory,  ii  gasometer,  a 
railway  shed,  an  elevator,  need  not  challenge  the  architect  in  vain  to 
piddntre  effects  of  fitness  not  entirely  iiu^onsistent  with  the  re(|uirenients 
of  art.  Indeed,  the  engineer  himself,  with  a.xioms  or  nia.xims  nt  art, 
has,  in  the  evolution  of  the  roof-truss,  the  locomotive,  and  many  Indus 
trial  niachines,  succeeded  in  satisfying  ideals  of  beauty  in  the  very  proc- 
ess of  making  them  powerful,  com|)ai'l,  and  economical  of  luaterial  and 
space.  The  mudern  steel-armored  war-ship  has  already,  in  this  early 
stage  of  its  rapid  development,  8U>  ituted  for  the  ideas  of  tnarifime 
beauty,  speed,  and  strength  which  prevailed  in  the  time  of  Xelson  and 
the  other  great  histu)ical  admirals,  ami  which  were  celebrated  in  :he 
songs  nf  iijbdin  and  Ciinipbell,  an  entirely  dilTereut  ideal,  baldly  less 
imposing,  Ihonghasyet  without  poetic  recognition.  Hut  the  evolntiun 
of  the  steel  trussed  liridge  has  aw' y(>t  satisfied  neither  old  ideals  uf 
beauty,  nor  has  it  made  new  ideals.  Its  essential  lines  are  drawn  in 
apparent  disregard  or  (*ontempt  fin'  grace  of  outline  or  elegance  of  de- 
tail. The  difTlenlty  seems  to  l)e  inherent  in  the  present  approved  struc- 
tural system  of  designing  horizontal,  straight,  open-trussed  girders  ur 
cantilevers,  resting  on  rigid  vertical  piers  of  masonry  or  iron,  without 
regard  to  any  other  considerations  excepting  those  of  statics.  The  eye 
requires  to  be  satisfied  as  well  as  the  trained  intelligence,  and  demandH 
not  only  grace  of  proportion,  but  a  certain  decorative  eniphaKls  expres- 
sive of   'special  fuuctious.    The  primitive  post  and  lintel  structure  of 


;icsTilK'ri(;s  is  i)i<:st(m. 


43 


stone  was  as  hopeless,  apparently,  as  iia  modern  derivative,  the  steel- 
trussed  biidge,  until  the  (irecks,  with  iiner?-iiig instinct  of  art,  C()riverte<l 
it  by  perfectly  rational  processes  into  that  ideal  expression  of  beauty 
wiiich  is  known  as  the  Doric  order.  This  Doric  order  is  a  structure 
whicJi  .depends  less  upon  subsidiary  decoration  than  upon  proportion 
for  its  unparalleled  success  as  a  work  of  art.  The  Parthenon  would  still 
be  lovely  without  the  sculptures  of  its  friezes,  metopes,  and  pediments. 
Its  columns,  reduced  to  dimensions  which  encumber  them  with  no  use- 
less brute  mass  of  material,  were  so  treated  with  entasis,  capital,  and 
tluting  as  to  expret-s  exactly  members  in  vertical  compression  ;  its 
lintels  were  so  snbdividiMl  as  to  draw  attention  to,  and  to  illustrate,  all 
their  f mictions  in  the  structural  scheme.  They  contained  no  features  of 
ciipricH  or  fancy.  Now  the  essential  qua.ities  of  tho  steel-girder  bridge 
iliffer  from  those  of  the  post  and  hotel  of  tlie  CJrecks  because,  in  the 
former,  the  striicturt!  of  the  lintels  permits  of  a  wider  spacing  of  tiie 
posts,  and  the  i)osts  have  assumed  the  dual  function  of  piers  for  vertical 
support  and  of  buttres.ses  to  withstand  the  horizontal  pressures  of  the 
stream  in  which  they  are  built  ;  the  lintels,  in  their  turn,  have  lost  their 
quality  a.s  compact,  M'lid,  homogeneous  masses,  have  been  resolved  into 
distinct  elements,  an  I  liavi-  become  a  comiilicateil  and  highly  artificial 
openwork  contrivance  of  light  steel  ineudiers,  which  in  their  dimensions 
and  articulii  >.s  have  been  .sucombiu-'d  in  tension  and  compression  as 
to  produce  a  r>irticture  u  ipable  of  sustaining  without  change  of  form 
not  oidy  its  own  weight  betvM-eii  bearing  [(oinis  l.ir  uj  >irt,  but  that  of 
moving  trains,  .•mil  of  bcni  ing  wii  liout  detriment  ubrations  and  wind- 
pressures,  and  the  expaiiiiou  ami  contraction  of  iir<  material  by  changes 
of  temperature. 

These  compound  lintels  or  tins.sen  are  in  tli'-niselves  triumphs  of  mind 
over  niattei'.  At  this  moment  they  e.\pre.ss  a  sta^'e  of  evolution  which 
has  l)een  in  process  for  a  century,  and  which  dor  iiless  w  dl  continue  to 
develop  in  directions  impossible  to  ant  icipat«>.  Tlioy  are  structures  not 
deilicated  to  the  immortal  goils,  like  '  post  and  lintel  in  the  Greek  tem- 
ples, the  decorative  clniracter  of  wlucli  was  largely  iiispirtHl  by  religious 
emotions,  but  devised  tit  meet  secular  and  practical  conditions  of  an  ex- 
ceedingly unpoetic  and  luiimaginative  cli.i  cter.  The  mind  of  the 
architect  appreciates  the  flue  economy  of  these  sensitive  and  com- 
plicated orgaidisms,  but  it  also  reogi  that  they  are  still  in  aclive 
process  of  development  ;  that  they  are  on  trial,  and  will  not  ivach  linal 
results  until  tlirtj  sitiill  /nice  nssittited  thitsr  CDiidition.i  nf  ijrace.  and 
lii'iiuhj  irhu'h  are  I'^m-ntial  to  comftli'tion.  It  is  evident  enoirgh  that  all 
the  featiu'es  of  perfection  in  aiunnils  have  been  very  gradually  evolved, 
by  survival  of  the  tittest  and  by  adaptation  to  use,  from  the  awkward 
and  luonstrous  shapes  of  the  antediluvian  period;  that  geological  ero- 
sion and  drift  liave  clothed  the  naked  rocks  with  beauty  ;  and  that  the 
whole  vegetablt^  creation  has  been  improved  by  art.  Nature  herself  is 
not  contented  with  inelastic  dogmas.  In  like  manner,  the  locomotive, 
the  steam-engine,  the  modern  war-slop,  have  all  l)econiu  objects  of  awful 


44 


BE  POKTIBITS. 


beauty,  not  becatise  of  tlie  imposition  of  unnecessary  features,  but 
hecaus(^  of  the  natural  and  reasonable  growth  of  their  essential 
structiif"'. 

If,  tlicreforc,  llio  ugly  character  of  the  present  ateel-trussed  liridge  is 
in  itself  a  proof  of  the  iitimaturity  of  the  seieiiee  which  has  produced  it, 
tlie  remedy,  of  course,  must  resitle  in  the  perfecting  of  the  science,  and 
this  process  of  perfecting  will  be  quickened,  if  beauty  is  rec&gnlze(i  !■■ 
engineering  as  it  is  in  architecture,  as  an  aim  and  not  as  an  acwident  of 
growth.  The  architect,  guides  and  hastens  this  progress  towards  the 
perfect  type  by  fundamentally  composing  liis  strnctmewith  a  view  to 
an  agreeal)le  proportion  of  its  parts  ;  in  detail  he  studies  to  emphasize 
the  special  and  important  points  of  his  structure  by  a  decorative  treat- 
ment which  shall  indicate  conventionally  the  character  of  the  work  ac- 
complished at  these  points.  It  is  true,  perhaps,  that  the  structural 
forms  of  materials  with  wliich  the  engineers  have  to  work,  especially  in 
bridge-building,  are  hardly  so  elastic  and  manageable  as  those  at  the 
cernuiand  of  the  architect  even  in  liis  simplest  and  most  severely  prac- 
tical problems;  but  it  is  none  the  less  true  that  the  training  of  the 
engineer  leads  him  too  often  to  an  absolute  disregard,  if  not  contempt, 
for  those  reflnemeiits  of  proporticn  and  outline,  and  for  all  those  delicate 
adaptations  and  adjustments  of  detail,  which,  though  perhaps  separately 
slight,  and  apparently  of  small  importance,  in  combination  tend  to  give 
distinction  and  a  character  of  fitness  and  grace  to  works  otherwise,  from 
the  point  of  view  of  art,  rudely  immature,  basely  mechanical,  un- 
necessarily and  insolently  ugly. 

Mr.  IleiM-y  .lames  says  that  the  French  talk  of  those  who  see  en  beau 
and  those  who  see  ini  laiil.  The  performance  of  the  modern  steel-bridge 
designers  would  certaiidy  seem  to  place  them  in  the  latter  category.  It 
is  no  less  certain  tliat  this  result  comes  not  from  temperament,  which  is 
natural,  but  from  training,  which  is  artificial.  The  severe  and  absolute 
conditions  in  which  the  bridge-builders  work  do  not  prevent  them 
either  from  great  differences  in  manner  and  method  of  design,  or  frotn 
frequent  and  utmecessary  extravagances  of  expenditure  ;  but  these  ex- 
travagances are  rarely,  if  ev('r,  lavished  in  the  services  of  beautj'  ;  be- 
caiisi'  the  cold  and  rarefied  atmosphere  of  science  and  mechanicnl 
utility,  in  which  they  are  accustomed  to  labor,  has  gradually  frozen  out 
(be  tin  T  natural  instinct  which  works  for  art  and  el«>gance  in  design. 
Ueauty  of  projiortion  has  often  been  proved  by  mathematics  ;  but 
mathematics,  when  it  has  lieen  allowed  to  be  the  only  elfirient  in  the 
development  if  a  jiroblem  of  construction,  has  never  accomplished 
beautiful  results.  Such  results  do  not  come  by  accident  in  any  work  of 
design,  but  liy  the  liberal  and  generous  observance  of  natural  laws. 
The  education,  therefore,  which  from  the  beginning  does  not  give  some 
recognitiiiii  to  grace,  proportion,  el(>gance,  as  essential  parts  of  construc- 
tion, must  be  misleading  and  one-sided,  and  cannot  lead  to  perfection. 
The  recognition  (f  these  ipialitles,  I  am  entirely  persuaded,  does  not 
necessarily  imply  any  sacrifice  of  jiractical  accuracy  In  design  or  of 


^ESTHETICS   IN   DESIGN. 


45 


mechanical  precision  in  worl<maiiship,  nor  need  it  affect  materially  that 
fine  economy  which  is  essential  to  perfection. 

Very  sincerely  yours, 

Henry  Van  Bri-nt. 

This  letter  of  Mr.  Van  Brunt's,  in  the  author's  opinion, 
gives  )i  very  just  uiid  unprejudiced  statement  of  the  status  of 
affairs  in  relation  to  the  development  of  bridge-building  from 
the  oesthetii;  point  of  view  ;  and,  in  calling  the  attention  of 
bridge-desiguera  to  their  lamentable  indifference  towards 
beauty  in  construction,  it  ought  to  be  the  means  of  inaugura- 
ting a  much-needed  reform  in  bridge-designing. 

In  thus  candidly  acknowledging  the  correctness  of  these 
allegations  of  the  architectural  profession  against  the  work  ot 
American  bridge-designers  the  author  wishes  it  to  be  under- 
stood that  he  considers  a  large  portion  of  his  own  past  work  as 
properly  subject  to  censure  ;  but  that  for  several  years,  more 
especially  since  he  severed  all  connection  with  the  contract- 
ing branch  of  bridge  building,  he  has  been  endeavoring  to 
reform  in  this  particular,  and  with  a  certain  amount  of  suc- 
cess, interspersed  perhaps  with  more  or  less  of  failure. 

The  principal  hindrance  to  the  progress  of  a;stlietic  reform 
in  bridge-building  is  liable  to  emanate  from  the  bridge-man- 
ufacturing companies,  who  have  been  so  accustomed  to  sub- 
mitting competitive  designs,  and  who  have  made  in  the  past 
so  much  money  thereby,  that  they  will  naturally  consider 
any  fundamental  innovation  of  this  kind  as  detrimental  to 
their  interests.  Nevertheless,  when  some  concerted  action  on 
the  part  of  bridge  specialists  is  iiuiugurated  with  the  object 
of  making  bridge  structures  more  sightly,  it  is  probable  that 
the  manufacturing  companies  will  be  far-sighted  enough  to 
recognize  that  their  true  interests  will  not  be  subserved  by 
offering  any  serious  opposition  to  the  proposed  reform.  Some 
obstruction  is  likely  to  come  from  managers  of  railroads, 
who  have  for  years  l)een  used  to  buying  their  bridges  as 
chcjiply  as  possible  without  any  regarcl  to  appearance,  and 
too  often  with  very  little  in  respect  to  constructive  excellence. 
It  will  devolve  upon  the  chief  engineers  and  the  bridge  en- 
gineers of  railroads  to  inlluenoe  the   managements  of   their 


iii 


DE    POXTIIJUS. 


lilies  so  as  to  incline  thoiu  towsirds  ti  more  fiivonible  consid- 
enilion  for  iipiieiimuce  when  deciding  upon  the  designing  and 
purchasing  of  their  bridges. 

But  the  moulders  of  public  opinion  in  respect  to  the  neces- 
sity for  a  due  considtMiilion  of  arcliilectur.il  ellecl  in  hriilge- 
building  must  of  necessity  be  the  independent  bridge  engi- 
neers of  the  country,  who  are  not  .so  much  infhienced  by 
monetary  motives  as  are  engineers  connected  witli  railways 
and  bridge  companies,  although  it  must  be  confessed  Ihiit 
some  of  the  most  prominent  bridge  specialists  are  the  greatest 
olTenders  against  the  principles  of  {esthetics. 

Tliere  is  a  general  impression  among  engineers  that  to  in- 
graft architectural  effects  upon  bridge  construction  will 
always  involve  the  necessity  for  an  increased  expenditure  of 
money  ;  but  this  notion  is  incorrect,  because  there  are  many 
large  and  important  liridges  in  the  United  Slates  which  could 
have  been  lieautitied,  and  at  the  same  time  cheapened,  witii- 
out  in  the  slightest  degree  impairing  tlicir  strength,  rigidity, 
or  efficiency,  by  simply  modifying  their  harsh  an  1  uncoui- 
promi-sing  lines.  It  reciuires  the  e.vpenditure  of  more  thought 
than  money  to  obtain  an  artistically  designed  bridge  ;  for  a 
little  money  will  go  a  long  way  in  producing  a  ilecoralive 
effect  upon  such  a  structure. 

Distinction  must  be  made  between  appropriate  and  inap- 
propriate, necessary  and  unnecessary,  and  expensive  and 
inexpensive  decoration.  For  instance,  while  it  is  always, 
proper  to  adapt  the  lines  of  a  structure  to  the  production  of 
llie  most  graceful  effect,  provided  that  in  so  doing  no  sacri- 
fice of  constructive  excellence  be  thereby  involved  or  extra 
expense  incurred,  it  would  often  be  injudicious  to  expend 
money  on  pure  decoration.  Tlic  builder  probably  cannot 
spare  the  money,  and  t!ie  location  of  Ilic  structure  may  be 
such  that  any  extra  expense  for  ornamentation  would  be  abso- 
lutely wasted.  If  a  bridge  is  to  be  located  where  it  will  be 
seen  constantly  by  many  people,  it  is  well  to  spend  extra 
money  to  make  it  sightly,  beautiful,  and  in  keeping  with  ils 
surroundings  ,  but  when  it  is  to  be  placed  in  a  dense  forest 
or  CD  a  sandy  desert  where  it  would  seldom  be  seen,  it  would 


.ESTUMICK    IN'    DESIGN 


l>e  folly  to  apoiid  nny  men!  on  its  coiistniclioii  than  is  culled 
for  by  the  eiigiiiecriiig  rc'iuireiiieiUs  of  the  conditioiis,  due 
ullowaiicc  being  made,  of  (our.so,  for  ii  pu,s>ibie  iieo[>ling  of 
the  forest  or  desert  in  the  not  veiy  distant  future. 

Tlie  style  of  ornamentation  for  a  bridge  should  always  bo 
iu  keeping  with  its  general  character;  thus,  in  case  of  a  light 
highway  bridge,  ornamental  portals  with  filigree  metal-work 
are  appropriate,  while  iu  large,  massive  railway  bridges  the 
ornamentation  should  be  of  a  coarser  and  bolder  character, 
commensurate  with  the  size  and  use  of  the  structure. 

The  author  is  a  Arm  believer  in  the  principle  that  true 
economy,  engineering  excellence  of  construction,  and  the  best 
architectural  effect  will  almost  invariably  be  found  to  a(  con> 
pany  each  other,  and  be  inseparable  in  the  designing  of  any 
biidgc.  Moreover,  any  l)ri(lg(!  built  with  due  consideration 
for,  tirst,  ellicicnc}',  second,  appearance,  and,  third,  economy, 
will  be  satisfactory  and  gratifying  to  not  only  the  trained 
expert,  but  also  to  the  general  engineer  and  railroad  man, 
and  even  to  the  public  ;  because  when  an  observer  notes  that 
in  such  a  stniclure  all  tlie  engineering  reciuirements  are 
properly  piovided  for,  that  there  is  no  evident  waste  of  mate- 
rial, and  that  all  due  advantage  has  been  taken  of  the  coudi 
lions  to  render  the  bridge  sightly  and  in  harmony  witli  its 
surroundings,  his  eye  will  of  necessity  be  pleased,  and  his 
inherent  sense  of  fitness  will  cause  hira  to  regard  the  structure 
with  a  feeling  of  pleasure. 

In  suggesting  that  "  if  a  steel  trussed  bridge,  economically 
and  wisely  consirucn-d  according  to  our  present  light,  olTends 
our  ideals  of  grace  and  beauty,  the  fault  perhaps  is  not  In  the 
structure,  but  in  the  rigidity  and  immobility  of  the  ideals 
which  have  been  established  by  conditions  long  since  out- 
grown in  the  progress  of  science,"  Air  Van  Brunt  has  prob- 
ably indicated  tlie  lines  of  convergence  of  engineering  practice 
and  architecturd  ideals  ,  for  while,  as  before  stated,  much 
can  be  done  with  most  bridge  designs  to  improve  them  with- 
out increasing  their  cost  or  affecting  their  efhcieuc}',  on  the 
other  hand  it  is  often  inipos  ible  for  an  engineer  to  modify  a 
bridge  design  so  as  to  meet  fully  the  critical  objections  of  a 


4S 


DK    PONTIHUS, 


good  arcliitert  without  introducing  fciiturcs  boUi  faulty  ntid 
expensive.  It  is  iiiglily  probable  liial  if  llie  engineer  will 
modify  liis  designs  as  uiucli  as  is  legitimate  to  meet  tlie 
fpsthetic  re(iuirenieiits  of  the  architect,  the  latter  will  gradually 
modify  the  rigitlity  of  his  ideals,  ami  will  see  lines  of  grace, 
beauty,  and  fitness  in  the  polygonal  outlines  of  trussed  bridges. 
Mr  Van  Brunt  himself  has  already  sliowu  this  to  be  true  by 
giving  his  unqualilicd  approval  to  the  architectural  elfei^l  ot' 
the  truss  outlines  in  the  draw-span  of  the  author's  bridge  over 
the  Missouri  Hiver  at  Omaha,  although  these  outlines  were  de- 
termined primarily  for  iitility  and  secondarily  for  appearance, 
and  notwithstanding  the  fact  that  there  is  no  attempt  at  even 
approximate  curvature  of  chords  in  the  entire  span. 

To  ret  agnize  and  acknowledge  the  deficiencies  of  modern 
bridge  designs  from  the  artistic  point  of  view  is  one  thing, 
l)ul  to  show  how  they  are  to  be  remedied  is  another  ;  because, 
while  it  is  easy  to  say  that  a  certain  stnu-lure  does  not  (.'onu! 
up  to  one's  ideal  of  grace  and  beauty,  it  is  very  difticull  to 
show  exactly  where  the  defects  are,  and  what  should  or  can 
be  done  to  remove  them. 

Notwithstanding  this,  the  author  will  now  endeavor  to 
establish  a  few  fundamental  rules  which,  if  followed,  ought 
to  correct  tlie  most  glaring  sources  of  ugliness  in  bridge 
designs  ;  then,  by  entering  more  into  detail,  he  will  try  to 
show  Inw  the  structures  may  be  decorated  appropriately  and 
inexpensively, 

The  architectural  treatment  of  bridge-designing  may  l»e 
divided  into  these  four  parts  ; 

Ist.  The  layout  of  .spans,  piers,  and  approaches. 

3d.  The  oui lining  of  each  span. 

3d.  The  decoration  of  each  spaa. 

4th.  The  ornamentation  of  the  entire  structure  by  tho  adop 
tion  of  elaborately  artistic  approaclies. 

In  respect  to  the  layout  of  spans,  piers,  and  approaches  for 
any  bridge,  there  is  one  governing  principle  which  should 
always  be  complied  with,  viz.,  that  the  entire  structure,  when 
ever  possible,  should  be  made  perfectly  symmetrical  al)OMt  a 
midc^le  plane. 


^STUETICrf   IN   DESIGN. 


49 


There  is  no  featuie  of  a  bridge  so  pleasing  to  tlie  eyes  of  all 
observers,  cultivated  and  ignorant  alijje,  as  perfect  symmetry 
in  tlie  layout  of  spans  ;  consequently  it  should  be  attained 
whenever  practicable,  even  if  some  extra  expense  be  involved 
thereby. 

Unfortunately  the  conditions  are  not  always  favorable  to 
perfect  symmetry  of  design,  for  tlie  bed-rock  will  often  dip 
rapidly,  and  thus  necessitate  the  use  of  spans  of  different 
lengths,  and  the  channel  of  the  river  often  refuses  to  keep  at 
midstream,  persisting  in  hugi^iiig  one  sliore.  In  sucli  cases  it 
becomes  -lecessary  to  do  the  best  one  can  with  the  \ui  favor- 
able conditions,  and  to  make  the  structure  sightly,  if  not  sym- 
metrical. If  there  be  a  draw-span  on  one  side  of  the  river,  it 
is  best  generally  to  niake  all  of  the  fixed  spans  alike.  Should 
each  span — because  of  the  gradual  shelving  off  of  tlie  bed- 
rock, and  for  the  sake  of  economy — lie  made  longer  as  tlie  bed- 
rock de('[)en8,  the  result  will  be  unsightly,  even  if  the  incre- 
ment of  span  lengtli  be  regular,  for  the  reason  that  to  an 
observer  there  is  no  apparent  motive  for  thus  diversifying  the 
spans. 

Any  divergence  from  symmetry  and  regularity  for  which 
there  is  a  self-evident  reason  produces  no  unfavorable  impres- 
sion upon  the  beholder,  although  it  maybe  sufticient  cause  for 
failure  to  excite  his  admiration  for  the  structure.  If  one  can 
see  at  a  glance  the  raison  d'etre  of  all  the  principal  parts 
and  peculiar  features  of  a  bridge,  his  sense  of  fitness  Avill  be 
satisfied,  and  his  general  impression  will  be  favorable  ,  but 
the  nearer  the  approach  to  perfect  symmetry  and  the  more 
artistic  the  outlines,  the  more  thorough  will  be  his  apprecia- 
tion of  the  general  effect  of  the  structure. 

In  making  a  study  of  the  teslhetics  of  a  bridge  design,  after 
determining  what  spans  are  applicable,  it  is  well  to  make  one 
or  more  layouts  on  a  large  .scale  on  the  brown  paper  used  in 
engineers'  offices  for  pencil-drawings,  indicating  tlie  circum- 
scribing lines  of  all  main  meinl)er8  to  scale,  anil  tinting  or 
filling  between  said  linos  with  pencil-shading  ;  then  tack  the 
paper  on  a  wall,  and  stand  off  at  various  distances  to  judge 
the  effect.    By  doing  th's  one  can  form  a  very  correct  o])inion 


50 


PK  roNTinrs. 


concerning  tlie  conipnmtive  merits  of  several  layouts,  and  can 
ascertain  where  and  how  any  particular  layout  can  be  im- 
proved. A  consiiltation  with  soveral  members  of  one's  office 
force  upon  the  architectural  features  of  the  various  designs 
will  often  result  in  an  improved  effect  ,  for  nothing  else  will 
bring  out  both  the  favorable  and  unfavorable  characteristics 
of  a  plan  like  discussion. 

In  the  o>il lining  of  each  span  a  great  deal  can  be  accom- 
plished towards  beautifying  a  structure,  aiul  lliere  is  no  l>etter 
way  to  study  the  geiuM-al  effect  of  any  proposed  outline  than 
the  one  just  indicated,  viz.,  laying  out  various  trusses  to 
scale,  tacking  the  paper  to  a  wall,  and  criticising  them.  It 
will  surprise  any  one  who  tries  this  method  to  sec;  how  quickly 
he  can  detect  the  slightest  variation  from  correctness  in  out- 
line, and  what  a  difference  in  efTeel  even  a  small  change  in  u 
truss  depth  will  produce.  It  wh-  in  this  w.iy  that  the  trusses 
of  the  Omaha  druw-spiin  were  proportioned  ,  aid  it  is  doubt- 
ful if  any  improvement  could  be  effected  in  their  outlines 
when  all  factors  involved  in  tlie  (|Ueslion  are  duly  considered. 
In  this  problem  there  were  but  three  points  to  determine,  viz., 
the  depths  of  truss  at  the  two  hips  and  the  depth  at  the  tower, 
for  the  number  of  panels  was  settled  by  economic  consideni- 
tions,  and  the  straightness  and  section  of  the  lop  chords  were 
necessitated  by  certain  (piestions  of  efliciency.  The  depth  at 
the  outer  hips  was  tinst  determined  by  tiie  requirements  for 
clearance,  rigidity,  and  appearance,  then  the  depths  at  the 
intermediate  hips  and  tower  were  settled  by  trial  and  discus- 
sion from  the  artistic  point  of  view,  due  attention  being  paid 
to  the  engineering  (jueslious  involved  by  the  various  iDcliua- 
tions  of  top  chords  and  incluied  inner  posts. 

In  determining  the  out.ines  of  a  span  these  few  elementary 
principles  are  to  be  borne  in  mind; 

1st.  There  is  nothing  so  ugly  in  a  bridge  as  parallel  chords 
unless  it  be  a  skew.  However,  for  spans  between  one  hundred 
and  twenty-tive  feet  and  two  hundred  feet  it  is  often  best  to 
use  them,  although  in  certain  cases  where  the  loads  are  great 
it  is  practicable  to  adopt  polygonal  top  chords  for  spans  con- 
siderably shorter  than  the  superior  'imil  just  uientioned. 


T.STIIETICS    IN    DKblGN. 


51 


2d.  While  it  is  pencrally  economiciil  of  mtiterinl  to  use  very 
long  imiiels,  no  such  extreme  length  should  be  adopted  ns 
would  involve  an  awkward  appearance  due  to  flatness  of 
diagonals. 

3d.  The  curvature  of  the  top  cliord  shoidd  be  made  as  great 
as  is  cousisteul,  wilij  a  proper  consideration  of  web  stiffness 
and  counterbracing. 

4lh.  When  It  is  practicable  in  Petit  trusses  to  curve  tlie  top 
chord  to  such  an  extent  us  to  make  too  small  the  in(;lination 
of  tlie  eiul-posis  to  the  horizontal,  it  is  permissible  to  let  the 
latter  extend  over  one  panel  oidy  and  to  make  all  the  main 
diagonals  extend  over  two  panels.  The  effect  is  ungraceful, 
however,  when  the  main  diagonals  occupy  one  panel  each 
near  llie  ends  of  the  span,  and  two  panels  each  elsewhere. 

5lh.  When  appearance  alone  is  in  question  trusses  very 
deep  at  mid-span  arc  desirable  ;  but  an  excessive  truss  depth  is 
conducive  to  a  reversion  of  bollom-chord  stress — a  condition 
which  has  either  to  be  avoided  or  provided  for  by  stiffening 
the  bottom  clionis.  In  extremely  heavy  bridges,  especially 
where  the  dead  load  is  unusually  great,  it  is  possible  that 
au  undue  consideraiion  for  economy  of  metal  might  cau.se  a 
designer  to  adopt  a  truss  depth  whidi  would  be  actually  too 
great  for  appearance .  but  this  is  not  likely  to  occur  very  often 
because  of  other  limiling  conditions. 

Gth.  Tljere  are  certain  limiting  relations  between  width  of 
bridge,  (le])th  of  truss,  and  length  of  span  which  lor  the  .sake 
of  good  effect  ought  not  to  be  exceeded.  I'sually  the  rules 
established  on  account  of  purely  engineering  questions  will 
prevent  these  limits  from  being  transgressed,  thus  proving  a 
maxim  which  tin;  author  lias  often  maintained,  viz.,  that  in 
any  design  any  violation  of  engineering  principles  is  also  a 
violation  of  good  taste  from  an  artistic  point  of  view. 

7th,  A  veiy  graceful  effect  can  be  obtained  by  placing  the 
lower  liorizonlal  struts  of  the  overhead  bracing  in  a  cylindrical 
surface  similar  to  that  which  contains  the  panel  points  of  the 
top  chords,  but,  of  course,  with  different  curvature. 

In  respect  to  the  decorutiou  of  each  span  of  a  bridge,  it  may 
be  stated  that  a  little  oruameulation  is  generally  much  better 


52 


I)K    r'ONTIBUS. 


than  a  great  ileal,  and  that  this  lillle  should  be  nppropiinte 
and  in  keeping  with  the  general  clmriicter  of  the  slructuio. 
A  prodigal  use  of  cheap  casl-iron  trimmings  at  a  portal  of  a 
steel  bridge  is  not  in  good  taste  ;  but  it  b  itjifectly  proper  to 
decorate  the  intersections  of  the  members  of  the  portiil  bracing 
by  plates  or  rosettes,  to  surmount  the  upper  horizontal  portal 
strut  by  iin  jpsthctically  designed  parnpet,  to  use  orniimentnl 
corner  brnckets  beneath  I  lie  lower  portal  strut,  to  employ 
fancy  name-plates  symmetrically  arranged,  and  to  place  orna- 
mental figures  of  proper  size  and  design  at  liie  hips,  pedestals, 
or  middle  of  inclined  end-posts.  It  is  also  permissible  to 
ornament  the  intermediate  transverse  vc  'icul  bracing  to  a 
slight  degree  b}'  rosettes  and  knee-brace  bi"  such  decoration 
should  be  applied  sparingly.  Again,  in  large  bridges  it  is 
proper  to  be  somewhat  extravagant  in  the  use  of  metal  iit  the 
portal  for  the  sake  of  appearance,  especially  as  such  metal,  if 
it  does  not  add  to  the  strength  of  the  bridge,  certainly  increases 
its  rigidity. 

The  ornamentation  of  viaducts  and  elevated  railways  is 
something  which  has  never  received  in  America  any  attention 
worth  mentioning,  as  is  proved  by  the  inherent  ugliness  of 
nearly  all  the  elevated  roads  of  our  great  cities,  and  the  i)aiu 
ful  plainness  of  our  railwjiy  trestles  throughout  the  country. 
It  is  principally  this  neglect  of  aesthetics  in  design  which  has 
created  such  bitter  ojiposition  on  the  part  of  the  property 
owners  to  the  building  of  elevated  roads  in  the  heart  of  the 
city  of  Chicarfo. 

Electric  lights  and  gas-fixtures  of  artistic  pattern  can  be 
made  great  aids  in  securing  a  pleasing  effect  in  designs  for 
bridges  and  viaducts  ;  and  at  night  a  well-studied  distribution 
of  incandescent  lights  can  be  made  to  i)roduce  a  brilliant 
appearance  at  the  portals  of  any  large  and  important  city 
bridge. 

Ornamental  handrails  are  also  of  great  service  in  decorating 
trestles  and  bridges,  especially  in  deck  structures,  where  these 
rails  can  be  built  in  the  form  of  a  highly  ornamental  parapet. 

Architectural  effect  in  bridge-building  seldom  derives  much 
aid  frv      paint,  for  the  reason  that  it  is  genenlly^  best,  on  ap 


/K8THKTIC8   IN    DKSIGN. 


58 


count  of  l)olli  convenience  and  good  taste,  to  use  but  one  color 
in  pidnliiif<  a  bridge.  A  j)r()per  choice  of  color,  however,  is  u 
niiileriiil  advantage  ;  and  it  is  correct  to  vary  the  color  in  cer- 
tain accessory  portions  of  tlie  structure,  such  as  machinery- 
liouses,  the  lettering  on  name-plates,  etc.  Some  engineers 
have  advocated  painting  tlu!  tension  and  compression  members 
of  dillerent  colors,  but  this  would  get  one  hito  difficulties  in 
spans  where  certain  strictly  tension-members  ate  made  stiff. 
Ornamental  figures  should  be  painted  of  the  same  color  ns  the 
rest  of  the  l)riilgc.  In  general,  it  may  be  stated  that  for  ordi- 
nary conditions  of  landscape  the  heavier  the  structure  the 
ligluer  should  be  the  color  of  the  paint  used,  for  the  reason 
that  if  a  bridge  has  an  appearance  inclining  toward  clumsi- 
ness this  objectionable  effect  can  be  'essened  by  reducing  the 
prominence  of  its  members;  while,  on  the  other  hand,  a  bridge 
which  is  of  such  an  extremely  light  and  airy  design  as  to  pro- 
duce an  ap  learance  of  weakness  can  be  made  to  look  stronger 
by  adopting  a  naint  of  dark  color,  and  thus  bringing  its  mem- 
bers into  greater  relief  in  respect  to  surrounding  objects.  With 
very  dark  backgrounds,  however,  it  will  often  be  advisable  to 
use  a  light-colored  paint  even  for  slight  structures,  so  as  to 
give  the  bridge  a  definite  outline. 

Ill  regard  to  the  ornamentation  of  bridges  by  the  adoption 
of  elaborately  artistic  approaches,  but  little  has  yet  been  done 
in  America,  the  reason  being  that  any  money  so  expended 
has  evidently  no  utilitarian  purpose,  and  con3e(piently  to  the 
eye  of  the  solely  practical  man  appears  to  be  entirely  wasted. 
Ill  Europe  it  is  customary  to  ornament  large  and  impoita-nt 
bridges  in  this  way  ;  and  the  time  is  coming  when  it  will  be 
the  practice  in  America  also. 

A  projier  proportioning  of  jueis  and  abutments  has  a  great 
deal  to  do  with  the  obtaining  of  an  artistically  designed  bridge; 
but,  unfortunately,  in  these,  even  more  than  in  the  super- 
structure, the  almighty  dollar  is  generally  the  ruling  influence 
in  the  design.  In  many  bridges  the  piers  do  not  seem  to  be 
massive  enough  for  the  spans  ;  and,  as  will  be  shown  in  Chap- 
ter XXII,  too  often  they  are  not  sufflciently  large  to  meet  cer- 
tain important  engineering  requirements,  which  are,  as  a  rule, 


54 


DE    PONTIHUS. 


iguored  by  the  average  designer,  and  occasionally  even  !)y  some 
who  consider  themselves  bridge  experts.  In  the  author's 
upinioM,  if  piers  and  al)utment3  be  adequately  designed  from 
an  engineering  point  of  view,  they  will  not  fall  far  short  of 
the  ideal  of  artistic  excellence. 

In  concluding  this  chapter  the  author  would  advise  each  )f 
his  readers  to  study  carefully  Chapter  XXVI  on  "The  Mi- 
thetic  Design  of  Bridgtis,"  by  David  A.  Molitor,  Esq.,  C.E.,  in 
Prof.  Johnson's  work  on  the  "  Theory  and  Practice  of  Modern 
Framed  Structures."  Although  most  of  Mr.  Molitor'.s  illus- 
trations are  necessarily  drawn  from  Europenn  practice,  there 
are  many  features  thereof  which  it  would  l)c  well  for  Ameri 
can  bridge-designers  to  adopt ;  notwithstanding  the  facts  that 
European  and  American  practice  in  bridge-building  are  fun- 
damentally and  essentially  dilleront,  and  that  American  engi- 
neers have  little  or  nolh'.ig  to  learn  from  their  brethren  across 
the  seas  concern  the  science  of  bridge  design.  From  an  artis- 
tic point  of  view,  however,  it  must  be  confessed  that  the 
average  American  bridge  is  inferior  to  the  average  European 
structure;  so,  while  it  is  advisable  thai  American  bridge-de- 
signers study  carefully  European  pr.iclice  in  respect  t.)  aes- 
thetics, they  should  be  caulicMis  to  avoid  tlioiightless  imitation; 
because  decorative  features  which  are  appropriate  to  the  heavy, 
nuissive,  and  costly  bridge.s  of  Kurope  would  be  out  of  jjlare 
when  engrafted  on  the  light,  airy,  and  ecoiiomic  structures 
that  are  cliaracteristic  of  American  engineering  practice. 


CHAPTER  V. 


CANTILEVER  BRIDGES. 


Theue  seems  to  be  a  notion  prevalent  among  the  uninitiated 
(engineers  tfX)  often  iachu'.ed)  thai  there  is  some  inherent  virtue 
in  cantilever  bridges  which  renders  tbeni  superior  to  ordinary 
structures,  in  what  parliculars,  however,  the  said  uninitiated 
are  ot  often  able  lo  state,  although  they  generally  claim  that 
it  is  iu  economy. 

This  notion  is  entirely  erroneous  ;  fov  cantilever  bridges  are 
always  inferior  in  rigidity  to  bridges  of  sinii)ie  truss  spans, 
and,  exc'.p;ing  for  (cnaiu  peculiar  conditions,  are  also  always 
more  e.<pensive.  These  exceptional  conditions  are  but  two, 
viz.,  deep  gorges  to  I)e  crossed  by  single  spans,  and  the  im- 
practicubility  of  using  false  work  because  of  danger  from 
wasliout. 

If  there  be  aasumed  a  river  crossing  of  very  great  length,  'n 
which  the  bed-rock  is  approximately  horizontal  and  wiiere  the 
conditions  alfectiiig  erection  are  not  iiiiusually  dangerous, 
there  is  no  possil)le  layout  for  a  caiildever  l)ridge  whicii  will  be 
as  inexpensive  as  a  structure  consisting  of  simple  truss  spans 
of  equal  length,  provided  that  the  said  length  be  the  most 
e(!oiionHC  one  possible.  'I'iiat  this  fact  is  not  generally  known 
is  |)roved  by  the  occasional  building  of  a  cantilever  bridge  iu 
a  place  where  the  conditions  do  not  r;ili  for  one.  For  instance, 
there  v  IS  no  good  reason  wlratsojvcr  for  making  the  great 
Poughkei'psie  Hridge  a  canlihiver  structure,  becatise  by  using 
tlie  same  number  of  piers  ;ind  making  all  the  spans  alike  the  cost 
of  'lie  substructure  would  not  have  been  at  all  iiu-reased,  but 
probably  diminished,  while  the  weight  of  metal  in  the  super- 
structure and  towers  would  have  been  lessened  materially.  It 
is  true  there  nuiy  have  been  a  little  saving  in  cost  of  false  work, 

55 


56 


HE   PONTIBUS. 


but  as  the  materials  could  have  been  used  several  times,  it 
could  uot  have  been  large;  while  to  partially  oltsel  it  there  is 
tlie  extra  cost  of  the  adjusting  apparatus  and  the  greater  cost 
of  erocliou  due  to  dehiys  in  making  the  central  connections. 
Moreover,  alternate  simple  spans  could  have  been  erected  with- 
out falsework  by  the  expedient  adopted  by  the  author  for 
several  Japuiiese  bridges,  which  expedient  will  be  described 
subsequently  in  this  ciiapter. 

There  is  a  small  cantilever  bridge  in  Philadelphia  close  to 
the  Pennsylvania  Railroad  where  it  approaches  the  depot, 
which  as  a  caut'lever  h.is  absolutely  no  raison  d'etre.  It  makes 
the  observer  think  that,  before  it  was  built,  some  of  the  city 
fathers  felt  that  Philadelphia  would  be  beiiiud  the  times  if 
siie  (lid  not  have  a  cantilever  Ijridge  of  some  kind  or  other, 
and  tiiat  they  erected  this  one  in  consecpience. 

Other  illustrations  of  unnecessary  cantilevers  could  be 
quoted,  but  it  would  be  a  useless  task  to  carry  the  illustration 
farther. 

If  a  deep,  narrow  gorge  with  rocky  sides  has  to  be  bridged, 
the  cantilever  construction  will  often  prove  economical  for 
two  reasons:  first,  the  main  piers,  being  small,  are  compara- 
tively inexpensive  J  and,  second,  the  cost  of  falsework  will 
bo  almost  entirely  eliminated,  only  a  small  amount  thereof 
being  used  for  erecting  the  anchor  arms. 

Again,  if  a  stream  is  to  be  bridged  where  it  is  impossible  to 
put  in  falsework,  or  where  tiiere  would  be  danger  of  its  being 
washed  out  in  case  it  could  be  put  in,  the  cantilever  will 
prove  an  economic  desig'i,  although  in  certain  cases  the  canti- 
lever arch  design  described  in  (Jhapter  VI.  may  bo  atill  more 
economical  and  possibly  more  rigid.  This  last  feature,  how- 
ever, will  depend  somewhat  upon  the  character  of  the  arch 
adopted. 

That  a  cantilever  bridge  is  less  rigid  and  deflects  more  ver- 
tically than  a  simple  spaa  bridge,  no  one  who  has  examined 
both  types  of  structure  under  load  and  who  has  measured  the 
vertical  defkiclions  can  well  deny;  nevertheless  this  compara- 
tive lack  of  rigidit}'  is  no  great  detriment  or  weakness,  and 
should  not  be  allowed  to  militate  against  the  building  of  a 


CANTILEVER   BRIDGES. 


57 


properly  designed  cimtilever  bridge  whore  the  conditions  call 
for  such  a  structure.  Compared  with  a  suspension  bridge,  a 
cantilever  bridge  is  rigidity  itself.  But,  again,  this  is  uo  rea- 
son for  condemning  in  toto  suspension  bridges,  which  have 
their  legitimate  place  in  engineering  construction,  viz.,  where 
either  an  extremely  long  span  is  necessary,  or  where  a  cheap 
highway  bridge  over  a  wide  river  is  required. 

There  is  ])ut  one  kind  of  steel  structure  in  which  the  canti- 
lever is  more  economical  of  metal  than  the  simple  span,  viz., 
roofs  supported  on  steel  columns,  as  in  train-sheds  and  work- 
shops. The  reason  for  this  economy  is  the  shortening  of  the 
spans  and  the  ignoring  of  the  etYects  of  reversion  of  stress 
when  proi)ortioniiig  members.  The  latter  is  legitimate  within 
certain  limits  because  of  the  in  frequency  or  improbability  of 
such  reversion. 

Cantilever  bridges  being  of  such  an  unusual  type,  and  their 
use  with  very  few  exceptions  dating  back  only  about  twenty 
3'ears,  but  little  elTort  has  yet  been  made  to  systematize  their 
designing  or  to  investigate  their  economic  features.  The  only 
paper  of  any  real  value  on  the  subject,  w  Jiicii  lias  come  to  the 
author's  notice,  is  one  l)y  Prof.  Edgar  Marburg,  j)\il)lished  in 
the  Procei'ditKjH  of  ihe  Engineers'  Club  of  Philadelphia  for 
July,  1896.  This  paper  is  an  excellent  one,  but  it  really  does 
not  .settle  any  im])ortant  point  concerning  the  economic  rela- 
tions of  sjjan  leiigtljs,  for  its  mathematical  investigations  are 
rather  cr»ide  approximations. 

As  the  author  has  lately  in  his  practice  accumulated  a  mass 
of  data  concerning  weights  of  metal  in  cantilever  bridges,  he 
lias  had  his  assistant  engineer,  Mr.  lledrick,  extend  Iuh  calcu- 
lations not  only  so  as  to  determine  all  the  economic  relations 
of  cantilever  bridges,  but  also  so  as  to  prepare  i>ercentage 
curves,  by  using  which  the  total  weight  of  metal  in  any  canti- 
lever bridge  of  any  ordinary  type  can  be  found  very  (juickly 
ami  with  considerable  accuracy. 

Before  proceeding  to  present  tliese  results,  though,  several 
other  matters  will  receive  consideration. 

In  no  work  on  l)ridges,  that  the  author  has  ever  seen,  has 
there  been  given  a  statement  of  the  various  stresses  for  which 


58 


DE   PONTIftUS. 


the  several  spans  of  a  cautilever  bridge  should  be  figured  ;  80 
such  a  tabulation  is  herewith  presented. 


Stresses  in  Susi'knded  Span. 

First.  Dead-load  Stresses. 

Second.  Live-load  Stresses. 

Third.  Impact-load  Stresses. 

Fourth.  Diiecl  AViuil-load  Stresses. 

Fifth.  Transferred  load  Stresses. 

Sixth.  Erection  Stresses  from  Dead  Load. 

Seventh,  Erection  Stresses  from  Wind  Load. 

Stresses  in  Cantilevkr-arms. 

First.  Stresses  due  to  Dead  Load  on  Suspended  Span. 

Secon  I.  Stresses  due  to  Live  Load  ou  Suspended  Span. 

Third.  Stresses  due  to  Lupact  L  )ad  on  Suspended  Span. 

Fourth.  Stresses  due  to  Wind  Load  on  Suspended  Span. 

Fifth.  Stresses  due  to  Transferred  Load  on  Suspended  Span. 

Sixt^i.  Stresses  due  to  Erection  of  Suspended  Span  and 
caused  by  the  Dead  Load. 

Seventh.  Stresses  due  to  Erection  of  Suspended  Span  and 
cau.sed  iiy  tlie  Wind  Load, 

Eighth.  Stresses  due  to  Dead  Lo.id  on  Cautilever-ann. 

Ninth.  Stresses  due  to  Live  Loud  on  (,'anlileverarni. 

2\'Hth.  Slres.ses  due  to  Liipact  Load  on  (yantilever-arni. 

Eleventh.  Stresses  due  to  Wind  Load  on  Cantilever  arm 

Twelfth.  Stresses  due  to  Transferred  Load  on  Cantilever- 
arm. 

This  load  affects  oidy  the  niaiti  inclined  posts  over  piers. 

Stresses  in  Anchor-arms. 

First.  Stres.ses  due  to  Dead  Load  on  Suspended  Span. 
Second.  Stresses  due  to  Live  Load  on  Suspended  Spau. 
Third.  Stresses  due  to  Lupact  Load  on  Suspended  Span, 
Fourth.  Stresses  due  to  Wind  Load  on  Suspended  Span. 
Fifth.  Stresses  due   to    Transferred   Load    ou    Suspended 
Spau. 


CANTILEVRU    BIIIDGKS. 


59 


Sij;ih.  Stresses  due  to  Erection  of  Suspended  Span  and 
caused  by  tlie  Dead  Load. 

Seventk.  Stresses  due  to  Erection  of  Suspended  Span  and 
caused  by  the  Wind  Load. 

Eighth    Stresses  due  lo  Dead  Load  on  Cantilever-arm. 

NitiUi.   Stresses  due  to  Livi;  Load  on  Cantilever-arm. 

Tenth.   Stresses  due  to  Iiu|)aet  Load  on  Cantilever-arm. 

Eleventh.  Stresses  due  to  Wind  Load  on  Cantilever-arm. 

Twelfth.  Stresses  due  to  Dead  Load  on  Anclior-arm. 

Thirteenth.   Stresses  due  to  Live  Load  on  Anelior-arm. 

Fourteenth.  Stresses  due  to  Impact  Load  on  Anchor-arm. 

Fifteenth    Stresses  due  to  Wind  Load  on  Anchor-arm. 

Sixteenth.  Stresses  due  to  Transferred  Load  on  Anchcrarin, 


STKE88K8   IN    ]\LviN    CeNTHAL   SpANS, 
CUOIIU   STUK88KS. 

Firftt.  Stresses  due  to  Dead  Load  from  both  Suspended 
Spans  and  Adjacent  Cantilever-arms. 

Second.  Stresses  due  to  Live  Load  covering  both  Suspended 
Spans  and  Adjacent  Cantilever-arms. 

Third.  Stresses  due  to  Lnpact  for  tlie  latter  case. 

Fourth.  Sircsses  due  to  Wind  lioad  on  both  Suspended 
Spans  and  both  Adjacent  Cantilever-arms. 

Fifth.  Stresses  due  to  Transfene<l  Load  on  both  Suspended 
Spans. 

Sirtli.   Stresses  due  to  Dead  Load  on  ]\Iain  Central  Span. 

S'venth,  Stresses  due  to  Live  Load  on  Main  Central  Span. 

Fif/hth.  Stresses  due  to  Impact  Load  on  .Main  ('entral  Span. 

Ninth.  Stres.ses  due  to  Wind  Load  on  Main  Central  Span. 

Tenth,  Stresses  due  to  'I'ransf erred  Load  on  Main  Central 
Span. 

Web  Stresses. 

Firfit.  Stresses  due  to  Dead  Load  on  1  oth  Suspended  Spans 
and  both  (Jantilevcr-arms. 

These  will  be  zero  for  a  symmclrieal  structure. 

Second.  Stresses  due  to  Live  lioad  on  one  Cantilever-arm 
and  oiu!  adjoining  Suspended  Span. 


CO 


DK   PONT  I  BUS, 


Tliis  loading  produces  a  constant  shear  from  end  to  end  of 
Main  Central  Span. 

Third,  Slres.'ses  due  to  Impact  from  last  load. 

Fourth.  Stresses  due  to  Transferred  Load  on  one  Suspended 
j>pan. 

This  loading  produces  a  constant  shear  from  end  to  end  of 
Main  Central  Span. 

Fifth.  Stresses  due  to  Dead  Load  on  Main  Central  Span. 

8i.Hh.  Stresses  due  to  Advancing  Live  Load  on  Main  Cen- 
tral Span. 

Seventh.  Stresses  due  to  Impact  from  last  load. 

For  certain  conditions  some  of  these  stresses  will  not  need 
to  he  considered,  but  in  other  cases  they  will,  consequently  it 
is  necessary  to  insert  them  in  *iie  lists.  For  instance,  in  the 
cantilever  nnd  anchor  arms  the  sixth  and  seventh  items  will 
generally  be  found  to  have  no  intluence  on  the  sections  of 
members,  but  in  some  cases  they  will,  as  in  long-span  high- 
way bridges  with  light  live  loads. 

In  calculating  erection  stresses,  the  weight  of  the  traveller 
must  not  be  forgotten,  as  its  intluence  on  such  stresses  is  by  no 
means  inconsiderable. 

The  combination  of  the  various  stresses  retpiires  both  judg- 
ment and  care,  for  some  loads  may  or  may  not  act  together, 
and  some  produce  tension  while  others  produce  compression 
in  the  .same  member.  Again,  distinction  must  be  made  be- 
tween groups  of  stresses  with  and  those  without  wind-stresses, 
so  as  to  use  the  different  intensities  of  working-stres.ses  given 
in  the  speciticntions  of  Chapters  XIV.  and  XVI.  It  would  be 
loo  tedious  to  give  here  t lie  various  combinations  of  stresses 
for  each  member  of  each  span  ;  but  it  will  suffice  to  8;iy  that 
the  computer  will  have  to  fiiul  for  each  main  member  in  the 
entire  bridge  the  greatest  tension  wIhmi  wind-stresses  are  in- 
cluded, the  greatest  tension  when  they  are  excluded,  the  great- 
est compression  when  they  are  included,  ami  the  greatest  com- 
pression when  they  are  ('.\(;luded,  taking  care  not  to  group 
together  any  stresses  that  cannot  e.\i>t  simultaneously. 

The  determination  of  the  proper  live  load  per  lineal  foot  for 
any  member  of  a  cantilever  l)ridge   is  oiu'  nupiiriiig  a  little 


CANTILEVEK    IJRIDGES. 


6L 


care,  the  rule  being  that  for  the  pie^c  considered  the  length  of 
span  to  be  used  in  applying  the  live-load  diagram  is  the  total 
length  of  structure  which  must  be  covered  by  the  moving 
load  in  order  lo  obtain  the  greatest  stress  in  the  said  piece,  ex- 
cepting only  the  suspended  span  and  tlie  main  central  span, 
for  which  the  live  loads  actually  imposed  are  to  be  treated 
exactly  like  those  of  simple  spans.  Of  course,  the  impact  is 
to  l)e  figured  for  llie  length  of  structure  that  must  be  covered 
by  the  live  load  to  produce  the  greatest  stress  in  the  piece  un- 
der consideration. 

Some  young  engineers  have  an  idea  that  the  finding  of 
stresses  in  cantilever  bridges  is  a  complicated  matter.  On 
the  contrary,  it  is  very  simple,  as  every  stress  can  be  deter- 
mined by  the  ordinary  prin(;iples  of  statics  and  very  readily  by 
the  use  of  graphics.  Although  the  work  is  simple,  it  is  some- 
what long  and  tedious,  as  is  evident  from  the  preceding  lists 
of  stresses.  The  computer  is  ad  viscvl,  when  finding  the  stresses, 
not  to  try  to  group  the  loadings  any  more  than  they  arc 
grouped  in  tlie  said  lists,  for,  if  lie  docs,  he  will  probably  have 
to  separate  them  while  making  his  combinations. 

In  respect  to  combinations  of  stresses  during  erection,  there 
will  be  ao  necessity  for  increasing  the  sections  pro[)ortioned 
for  other  combinations,  provided  they  arc  as  large  as  those 
required  by  the  said  erection-stress  coml)inations  with  the  in- 
tensities given  in  tiie  specifications  (Chapter  XIV.)  for  com- 
binations that  include  Avind-stresses,  viz.,  intensities  thirty 
per  cent  higiicr  than  those  for  conUjinatious  without  wind 
stresses. 

Cantilever  bridges  may  be  made  either  through,  deck,  or 
half  througli;  but  a  combination  of  deck-spans  for  the  anchor- 
arms  and  a  tlirougii-span  for  the  cantilever-arms  and  sus- 
pended span  is  awkward-looking  and  unsightly.  There  is  a 
structure  of  this  type  across  the  St.  Lawrence  Itiver,  near 
Montreal. 

It  is  no  easy  matter  to  give  an  artistic  effect  to  a  cantilever 
bridge;  nevertheless  it  is  generally  wiihin  the  realms  of  pos- 
sibility to  do  so,  althougli  it  must  be  confessed  that  most  of 
the  cxistinj^  structures  of  this  type  are   uncomj)romisingly 


6-' 


DE    I'ONTIBUS. 


ugly.  If  a  convex  upward  curvo  can  bo  placcnl  in  the  top 
chord  of  the  su.speuded  span,  so  as  to  reverse  at  (he  ends  into 
a  concave  iijiward  curve  on  the  cant  Hover  arm,  a  graceful 
effect  will  be  obtained  ;  but  the  design  generall}-  will  not  be 
economical  for  erection  on  account  of  the  large  erection- 
stresses  near  the  point  of  suspension.  The  author  luis  made  a 
de.sign  on  these  lines  for  a  proposed  MOO  ft.  span  liiglivvay 
bridge  to  cross  the  Mississippi  River  al  St.  Louis  ;  and,  as  the 
suspeiuled  span  would  be  eroded  on  falsework,  there  is  no 
want  of  economy  involved.  The  layout  with  all  the  m:iin 
members  dra,wn  t(»  true  scale  lias  a  very  pleasing  effect. 

In  long  spans  like  this  it  licconu's  nec(s.sary  to  widen  the 
cantilever  and  anchor  arms  uniformly  fromeniis  to  main  pi>.r.s, 
so  as  to  obtain  the  requisite  rigidity  for  resisting  wind-pressure 
and  .so  as  to  keep  the  wind-stro.sses  in  bottom  chords  wliliin 
reasonable  limits.  It  seldom  |)ays,  however,  to  build  the 
trusses  of  these  arms  in  planes  inclined  to  the  verlical, 
principally  l)ecause  of  the  complicated  shop-work  involved. 

The  author  has  lately  had  occasion  to  design  a  number  of 
large  britlges  for  a  proposed  bran(di-line  of  the  Nip|)on  Kail- 
way  of  Japan.  The  line,  which  will  be  about  one  hundred 
miles  long,  is  to  follow  the  course  of  a  mountain  torrent  that 
rises  from  twenty  to  twenty-live  feet  in  two  or  ihree  hours, 
and  attains  in  places  a  depth  of  water  exceeding  one  hundred 
feet  with  a  total  rise  of  sixty  feet.  Of  coinse,  falsework  can  be 
employed  for  these  bridges  only  to  a  very  limited  extent,  hence 
it  was  necessary  to  resort  to  the  u.se  of  the  cantilever.  Thr^  e  of 
the  eight  structures  were  designed  as  ordinary  cantilevers,  two 
as  simple  truss-bridges,  and  tlu-(.e  as  cantilevers  during  erec- 
tion and  simple  spans  afterwards.  The  last  style  of  bridge  is 
very  economical  of  both  metal  and  money,  and  will  bear 
fiuther  investigation  and  extension,  so  as  to  be  made  appli- 
cable to  crossings  where  the  ordinary  cantilever  bridge  would 
otherwise  be  adopted.     Its  modus  operandi  is  us  follows  ; 

At  each  side  of  the  river  there  is  erected  on  false  work  a  sim- 
ple span  having  its  chords  and  certain  of  its  web  members  (or 
for  short  spans  all  of  them)  stiffened  for  eirciiou  stre.-ses. 
Then  over  each  pier  is  built  a  toggle  consisting  of  horizontal 


CANTILEVKIl    HKIDCJIiS. 


6;j 


P 


u 

y 

lO 


ti|»l)er-chord  eyebais  mid  adjustiiLIe  veitirals,  by  means  of 
wliicli  one  lialf  of  the  central  spun  is  cunliUivered  over  tlie 
stri-am  to  ineol  the  otiior  liidf,  after  winch  the  toggles  are  to 
he  removed.  This  mctliod  of  ereclion  can  be  understood  by 
reference  to  the  diagram  in  Fig.  1. 


\M^m^ 


i^'^ 


Fio.  1. 

One  of  the  three  oases  nuintloned  had  rather  peculiar  con- 
ditions, which  necessitated  the  adoption  of  another  expedient. 
About  nndstream  there  is  a  narrow  rocky  island  that  reaches 
to  about  the  elevation  of  extreme  high  water.  Near  the  edges 
of  this  island,  as  shown  in  Fig.  2,  will  be  built  two  small  piers, 


Fia.  2. 


each  of  which  will  support  one  end  of  a  long  8p:in.  Between 
the  end  shoes  will  run  a  temporary  strut,  and  from  each  ped- 
estal will  spring  a  temporary  post  to  support  the  temporary 
top-chord  eye  bars  that  run  from  hi[)  to  hip.  The  rectangular 
panel  is  braced  with  Iciiipora'-y  adjustable  diagonals,  and  the 
top  chord  is  hinged  at  the  nuddle  and  connected  to  tlie  pedes- 
tals by  other  temporary  adjustable  rods.  The.»e  two  sets  of 
adjustable  rods  permit  of  liie  raising  or  lowering  of  one  span 
at  a  time.  By  means  of  this  device  more  tlian  one  half  of 
each  span  can  be  cantilevered  out  to  meet  the  remainder  there- 
of, which  will  be  erected  on  falsework. 


u 


DE    PONTIBUS. 


It  is  intended  to  erect  the  cantilevercd  portions  of  all  three 
bridges  with  their  ends  higher  thiin  they  will  be  in  their  final 
position,  so  tliat  no  raising,  but  only  lowering,  of  tlie  weight 
of  the  arms  by  tlie  toggles  will  be  necessary.  The  author  is 
of  the  opinion  that  these  toggles  will  work  much  more  easily, 
and  will  prove  in  the  end  less  costly,  than  the  wedges  used 
for  adjustment  in  the  erection  of  the  Red  Hock  Cantilever 
I'  !ge,  a  description  of  which  was  given  by  Samuel  M. 
Rovve,  31.  Am.  Soc.  C.  E.,  in  the  Transactions  o(  that  Society 
for  1891 

In  one  of  the  throe  true  cantilever  bridges  for  the  proposed 
Japanese  railroad  an  expedient  lias  l)cen  adopted  by  the 
author  which  ma}' lie  worthy  of  description.  One  approach 
to  the  structure,  as  shown  in  Fig.  3,  is  through  a  tiinuel  end- 


\TxKK^/KKfWlX\fc^s^ 


Fig.  3. 

ing  in  the  face  of  a  vertical  wall  of  rock.  It  was  at  first 
intended  to  use  this  rock  in  lieu  of  one  anchor-arm  of  an 
ordinary  cantilever  by  letting  the  main  i)osts  lie  close  to  its 
vertical  face  and  tying  the  top  chords  well  back  into  its  ma.ss  ; 
Imt  a  study  of  the  contours  of  the  rock  showed  that  it  dipped 
ofif  to  one  side  of  llie  line  in  such  a  way  as  to  render  such  an 
anchorage  of  uncertain  strength,  so  it  was  decided  to  increase 
the  lengths  of  the  suspended  span  and  far  cantilever-arm 
sufficiently  to  cut  out  the  near  cantilever-arm,  and  thus  let 
the  end  of  the  suspended  span  roll  on  two  small  pedestals  at 
the  luouth  of  the  tunnel.  Five  eighths  of  this  span  will  be 
erected  by  toggles  fastened  into  the  rock,  and  tlie  reuiaiidng 
three  eighths  will  be  cantilevcred  out  also  by  toggles  from  the 
end  of  the  far  cautilever-arui.  This  method  requires  more 
metal  than  does  the  one  first  contemplated  ;  nevrrlheless  it  is 
the  cheapest,  everything  considered,  that  can  be  adopted. 
The  rock-anchorage  js  amply  .strong  for  the  dead-load  pulls 


CANTlLLVhIt    i;Ull)<ih.>>. 


6J 


on  il  (liiriiijr  erection,  iilthoiiL'li.  as  before  staled,  it  is  uot  silt- 
liciciitly  ielial)le  for  resisliim  tlit!  e/rtcts  of  live  lomls. 

Tlie  liest  nulliod  of  atlatiiiiig  the  suspended  span  of  an 
oniinaiy  cantilever  bridge  is  by  lianuers  from  inclined  end 
posts  on  the  cuntilever-arnis.  For  such  suspenders  narrow 
eye-bars  sliould  be  used  ;  and  it,  is  generally  better  to  hinge 
Ihein  at  the  middle.  This  is  because  they  are  subjected  to 
transverse  bending,  due  to  longitudinal  expansion  and  con- 
traction of  suspended  span  from  both  changes  of  temperature 
and  the  appli(;alion  and  icnioval  of  the  live  load.  Narrow 
bars  can  spring  slightly  without  being  overstrained,  and  a 
rotation  of  the  eyes  ou  the  pins  will  thus  be  prevented.  Such 
a  rotation  would  eventually  enlarge  the  eyes  and  cut  notches 
into  the  pins,  necessitating  for  some  futun;  time  e.vpenslvt 
repairs. 

A  8us]iended  span  thus  hung  is  free  to  move  longitudinally 
under  thrust  of  train,  but  its  ends  are  tightly  held  in  a  lateral 
ilirection,  so  that  all  wind  loads  are  carried  i)r()perly  to  the 
bottom  (!hords  of  the  cHUlilever  arms ;  aiul  excessive  longi- 
tudinal motion  is  prevented  by  the  continuity  of  the  track. 

In  cantilever-arms  it  is  better  and  moie  economical  to  use 
inclined  posts  as  wtll  as  vertical  ones  over  the  piers,  so 
that  the  various  loads  will  be  canied  more  directly  to  the 
masonry.  To  insure  the  travel  of  the  wind  stresses  down  the 
transverse  bracing  between  these  inclined  i)osts,  instead  of  up 
to  the  apex  of  the  top  chord  and  down  the  bracing  between 
the  vertical  posts,  the  author  leaves  out  oik;  pair  of  diagonals 
of  the  upper  lateral  system  between  the  said  apex  and  the  tops 
of  the  inclined  posts.  The  same  expedient  is  used  also  for 
the  anchor-arms  and  between  the  hips  of  the  suspended  span 
and  the  canlilever-arins. 

All  bracing  between  oppo.site  veilical  posts  and  between 
opposite  inclined  po>ts  should  be  made  very  rigid  ;  and  in 
douliletrack  .structurts  all  the  sway-bracing  slnnild  be  i>ro- 
portioned  to  carry  as  a  Uw  load,  with  the  i)roper  allowance 
for  impact,  the  greatest  shear  which  can  come  upon  it  from 
loading  one  side  of  the  tloor  oidy. 

(Jreal  cure  is  necessary  in  designing  the  pedestals  over  tbo 


66 


DH    I'ONTinrS. 


inuiii  pit'is  80  lis  to  cjiiry  tl.c  loads  lioin  the  llircc  hciivy  posts 
to  tlie  iiiusonry  willioul  ovcrslniiiiiiig  any  of  lliu  rnotiil  in  tlio 
pedestal,  and  so  us  lo  distribute  Uie  total  prtss\ire  \iiiiforiiily 
over  tiie  masonry  bearini;.  L'ntil  recently  tlie  autlu»r  has 
adopted  pediistals  bidll  of  [)lales  and  sliapes,  but  has  lately 
deei(Jed  to  try  steel  castings,  us  the  i)ouud  price  for  these  has 
now  come  down  to  sonietliing  like  a  leasonable  figure.  The 
(liHiculty  iu  finding  room  for  the  proper  number  of  rivets  for 
Hi  ladling  together  llieir  component  parts  renders  built  petles- 
tnls  chimsy  and  expensive. 

The  anchorage  details  reipiire  special  care,  and  no  rules 
can  be  given  U>  govern  their  designing,  for  the  reason  that 
the  conditions  vary  for  all  crossings.  The  following  hints, 
tliougli,  nuiy  be  of  use  to  the  designer  : 

First.  The  anchor-bars  should  be  made  as  long  and  as  nar- 
row as  practicable,  and  should  be  divided  into  sliort  lengths 
by  pins,  for  tlio  sami;  re  ison  as  given  in  llie  case  of  the  sus- 
l)enders  of  tlie  suspended  span. 

Second.  All  anchorage  details  should  be  accessible  to  the 
paint-brush,  excepting,  of  course,  those  portions  of  the  bot- 
tom girders  wliich  are  buried  in  the  masonry.  This  result  is 
accomplislicd  by  leaving  wells  in  the  ancliorages  of  suflicient 
size  to  permit  tlie  passage  of  a  man  to  do  the  painting.  If 
these  wells  are  at  any  time  i)artially  filled  willi  water  tempo- 
rarily by  the  rise  of  the  stream,  no  harm  will  be  done,  pro- 
vided that  the  painting  of  the  metal-work  therein  be  always 
attended  to  pro|)erly. 

T/ii'rd.  Concrete  foi  ancliorages  is  always  i)referable  to 
masonry,  because  it  can  readily  be  made  to  lake  any  reipiiied 
form.  If  necessary,  its  exterior  can  be  protected  against 
alnasion  from  ice  or  drift  by  facing  with  granite  or  other  hard 
rock. 

Fourth.  There  sliould  be  an  iudepcudeut  anchorage  against 
wind-pressure,  obtained  by  sliding  surfaces  of  steel,  one  of 
each  pair  of  same  forming  purt  of  a  heavy  detail  which  is 
rigidly  attached  lo  the  bottom  of  the  end  floor-beam,  and  the 
other  forming  part  of  a  heavy  detail  that  is  anchored  lirml^ 
to  the  musoury. 


(.ANTir.KVKU    ItUIIKlKS. 


67 


Fiflli.  Th(!  tops  of  the  michor-picrs  should  be  Tiuuic  al)Ho- 
IiiUly  \val»'i-li<;lil  witlioiil  iiilcifciiiif;  with  tlic  loiiiiiUKlinnl 
<  xpan.sioii  of  tlic  anchor  arm,  .so  as  lo  i)rcvt'nt  nisliiig  of  Ihe 
iiit<!iiof  uictiil-work. 

Sixth.  TJie  net  wcij,'hl  of  masonry  in  any  unchorpier,  after 
(l«M]ii(;lini,'  the;  urciitost  huoyant  cUtMl  of  tlie  tlisi)la(»!(l  wattir, 
.should  bi>  Iwicc  as  great  as  the  inaxinuini  uplift  on  the  .said 
aiiciioi-picr,  when  tiie  eU'nct  of  impact  is  duly  included. 

A  few  observations  concerning  some  of  the  largest  caMti- 
lever  bridge.s  yet  built  may  be  of  service  to  the  reader  : 

Till'  largest  structure  of  this  type  in  the  world  is  the  bridge 
at  (^ueensferry  over  the  Firth  of  F()rlh,  the  main  portion  of 
wliich  consists  of  two  spans  of  1710  ft.  each,  with  central 
spans  of  yr)()  ft.  each,  and  two  anchor-arms  of  680  ft.  each. 
The  length  of  the  tower-span  over  ilie  centre  pier  is  200  ft., 
and  that  of  each  of  the  two  other  toweu-spans  is  145  ft., 
making  the  total  length  of  the  main  structure  5410  ft.  Tho 
design  for  this  bridge  and  a  complete  history  of  its  <'oii- 
slruction  are  given  in  a  .special  work  published  by  Engineer- 
in;/  (London). 

The  exceptions  whic;h  the  author  would  take  to  this  design 
are  as  follows  : 

Fir.st  The  suspended  s|>an8  are  just  about  one  half  as  long 
as  lli(!y  ought  to  be  foi-  both  appearance  and  economy. 

Second,  The  structure  should  have  been  made  piii-con- 
necled  for  both  case  of  erection  anil  certainty  of  stress  dis- 
tribution. 

Tliird.  A  single  .systc;*.".  of  cancellation  for  the  webs  of  the 
girders  would  have  been  more  scientiilc  than  the  double  sys- 
(iin  adopted,  and  would  not  have  been  any  more  expensive. 

Fourth.  The  structure  as  a  whole,  fronj  the  point  of  view  of 
American  i-ngineers,  was  unnecessarily  expensive 

On  the  other  hand,  though,  the  labor  Involved  in  both  the 
designing  and  buil.lin;^'  of  this  bridge  was  immense  ;  and  the 
successful  compljtioM  of  the  stru(;ture  is  a  great  credit  to  all 
concerned  in  its  dosigiung  and  construction. 

The  C'lutiiover  bridge  having  the  next  longest  span  is  the 
Lausdowne  Bridge  over  the  Indus  Kiver  at  Sukkur,  ludiu 


68 


I)K    roNl'IHU.S. 


Tt  consists  of  Ji  siiiirlt'  spun  of  8','()  f(.  willioul  iinclior-Jirnis, 
\}.iC  hitter  being  replaced  by  guys,  uiid  witli  ii  suspended  span 
of  200  ft.  Tiie  uppeanince  of  llie  bridge  is  bizarre  in  the 
extreme,  and  the  slnu;lure  is  economic  in  i;eillier  weigiit  of 
material  nor  cost  of  sliop  work.  Compared  witii  an  American 
bridge  of  llu;  .same  span,  capacity,  and  strength,  the  weiglils 
of  ineial  in  llie  820-ft.  span  only  would  be  about  iu  the  ratio 
of  unity  to  0.7r>. 

The  ci!.  itilever  having  the  ne.xt  l';:-.j;est  span,  viz.,  790  ft.,  is 
the  railway  britige  at  Mcm|>his  over  the  Mississippi  River. 
This  structure  is  botii  unsightly  and  uneconomical  of  uuiterial. 
Its  layout  of  spans  is  \infortunate  (but  the  War  Department, 
jtnd  not  the  designer,  is  lesponsible  tor  this),  and  the  truss 
depths  are  far  too  small  for  both  economy  and  a[)pearaijce. 

The  re.vt  longest  cantilever  span  is  that  of  the  lied  iiock 
Bridge  over  the  Colorado  River  on  the  Atlantic  and  Pacific 
R-dhvay.  Tliis  structure  consists  of  a  main  span  of  660  ft. 
and  two  anchor-arms  of  Ki.'}  ft.  each,  the  length  of  the  sus- 
pended span  being  830  ft.  Tlie  widtli  between  central  i)lants 
of  trjisses  is  15  ft.  ;  and  the  truss  tlepth  varies  from  55  ft.  for 
the  su.spended  span  to  101  ft.  for  the  vertical  posts  over  the 
main  j)iers.  As  tiie  author  is  liie  peison  responsibli;  for  its 
layouc,  his  criticism  thereof  will  not  be  of  much  valu'  Tlie 
bridge  was  designed  to  meet  certain  conditions,  economy  in 
first  cost  being  the  prime  reepiisite  ;  consctiuently  the  subjci  I 
of  a'sllietics  did  not  receive  great  consideration.  Engineers 
Hud  architects  ditfer  fundamcnlally  in  llieir  opinions  concern- 
iug  the  architectural  elTect  in  this  structure.  Some  approve 
its  appearancte;  other.s  characterize  it  as  harsh  in  its  outlines. 
Tlie  relations  between  lengths  of  su,spended  span,  cantilever- 
arms,  and  anchor-aruis,  and  those  of  width  and  depth, 
although  very  hurriedly  determined,  hav(!  since  been  found 
to  be  just  about  the  best  practic;ii)ie.  This  bridge,  as  before 
stated,  is  •■■::>cribed  ver}'  fully  in  the  Tranmctionx  of  tiie 
American  bociety  of  Civil  Engineers  Tor  1891. 

There  are  many  other  <'antile  "<'r  bridges  having  main  spans 
of  from  400  ft.  to  500  ft,  nr  more,  but  sjiace  will  not  permit 
thfiir  enumerfiliou. 


SI 

n 


(JANTII.F-VKfl   T5UID0KS. 


Many  expedients  have  Ixvii  used  to  connect  the  metal-work 
(if  tlie  juspeudcd  spans  of  cantilever  bridges,  and  considerable 
troubi''  f.as  often  been  experienced  in  doing  tbe  work,  owing 
to  variations  in  bolh  Icnglh  and  elcviition.  Tbe  author  is  of 
the  opinion  iliat  but  little  ditliculty  will  be  experienced  if  the 
following  precautions  be  taken  : 

First.  See  that  the  entire  triangulation  is  so  accurately  done 
that  there  will  be  no  jiossibility  of  an  error  exceeding  one 
i|Marler  of  an  inch  in  the  ilistunc-c  between  centres  of  pins 
over  main  piers.  A  perusal  of  Cliapter  XXIII.  will  show  that 
this  is  perfectly  feasible. 

Second.  See  that  extra  precautions  are  taken  l)y  the  inspect- 
ors during  tlie  manufacture  of  tiie  metal-work  to  insure  that  all 
lengths  of  main  member,  shall  be  absolutely  correct. 

Third.  See  that  the  tapes  used  in  shop  .and  tield  arc  of 
exactly  the  same  length. 

Fourth.  Use  toggles  like  those  described  in  this  chapter  for 
elfecting  the  adjustment. 

Fifth.  Arrange  to  have  the  meeting  ends  of  the  chords  a 
tritli'  high,  so  that  lowering  and  not  raising  will  be  necessary. 

Si.ith.  Arrange  matters  so  tiiat  when  the  eiuls  of  the  metal- 
work  come  together  they  will  be  a  trifle  apart  ratiier  than  tend- 
ing to  lap,  for  it  is  nuicii  easier  to  heat  the  chords  slightly  by  sus- 
pending beneath  thein  siicetsof  metal  containingslow  tiresthan 
it  would  l»e  to  cool  them  by  packing  ice  around  them  in  cloths. 

Referring  now  to  (lie  before-mentioned  special  investiga- 
tions nuide  b}'  Mr.  lledriok,  inc  (piestions  set  him  for  solution 
at  the  outset  theri'()f  were  the  following  . 

Firs/.  'I'lie  ratio  of  the  economic  length  of  suspended  span 
to  tJiat  of  the  total  o|)etMng, 

Seco7id.  Tlie  most  economic  length  of  anchor-arms  when  the 
total  lenizth  l)cfweeii  centres  of  :ii\cborages  is  given,  and  when 
the  main  piers  cat)  he  placed  wherever  desired. 

Third.  The  relations  between  the  weights  of  metal  in  die 
suspended  spun,  cantilever-arms,  anchor-arms,  anchorages, 
main  pedestals,  juid  nncliorspans. 

Fourth.  'IMie  best  proportionate  length  for  anchor  spans,  and 
the  comparative  weights  of  nu'tal  in  those  of  dilferent  lengths. 


to 


hK  I'ONTtias. 


Fiflh.  Tlie  nitio  of  weigliis  of  inetiil  in  cantilever  bridi^es  of 


to  tlu 


)le- 


l)ii(lge8  li 


tliu  siiint 


viirious  ly 
uiinilKT  of  spiius. 

Mr.  Ilt'diick's  luclliod  of  (U'tciniining  tlie  economic  functions 
was  t<»  (like  llic  dutii  on  liaiid  for  tlie  |)roposed  Japanese  caiili- 
ievcr  biidgea,  exact  weiglits  of  inetul  liuving  been  conii)Uted 
for  structures  of  I{"20-ft.,  400-ft.,  and  500-ft.  openings,  and,  i»y 
varying  tiie  layouts  so  as  to  use  longer  and  sliorlcr  suspciidctl 
spans  and  longer  and  shorter  ancliorarnis,  obtain,  by  at;tual 
designs  and  estimates,  the  weights  of  nielal  for  a  suUicient 
nundxr  of  layouts  to  indicate  the  desuc  1  mininui. 

In  deterniiiung  the  economic  length  of  susjKMidcd  span  for 
a  certain  opening,  the  Icnglh  for  the  ancliorarnis  was  tiri-t 
assumed  to  be  one  fourth  of  said  o|)ening,  then  the  total  weight 
of  metal  in  the  entire  bri<lge,  including  even  the  anchorages 
and  pedestaN,  was  figured  for  s"veral  cases  ;  and  the  lenglii  of 
suspended  span  giving  the  least  weight  of  metal  for  the  whole 
structure  was  found  to  be  about  thn^e  eighths  of  the  opeinng, 
altlioUirh  this  k'nglh  showed  only  one  and  a  half  per  cent 
advanlai^^e  over  the  case  where  the  ratio  wis  on<!  half.  Nnw, 
as  the  rigidity  of  the  entire  structure  ceitainly  increases  with 
the  length  of  ilie  Muspcndcd  span,  it  will  often  lie  found  best 
to  make  the  lenglii  of  the  latter  about  one  half  of  the  opening 
rather  than  three  eighths  or  any  smaller  ]>roportion.  On  ihe 
otlier  hand,  tliougli,  it  has  been  found  by  trial  tliat,  with  tht; 
thi(!e-eightlis  ratio,  there  results  a  more  sightly  layout  than 
can  b(!  obtained  with  the  one-half  ratio. 

Next  Mr.  lledrick  tabulated  the  various  component  truss 
and  lateral  weigiils  of  several  of  the  typical  cantilever  bridges 
designed  in  tin;  author's  ollice,  the  leading  diiiiensions  for 
which  are  given  in  the  following  table. 

P'rom  these  weights  he  constructed  the  curves  shown  on 
I*iate  X,  from  wliicli  can  be  found  the  total  weight  of  metal 
in  the  trusses  and  lateral  systems  of  any  three-span  cantilever 
bridge,  wlieii  the  weight  per  lineal  foot  of  tlw;  trusses  and 
laterals  in  the  suspended  span  is  known.  Tiiis  weight,  by  liic 
way",  is,  on  tli<r  average,  (Mglit  per  cent  gi(.'ater  than  that  for 
un  ordinary  simple  span  of  the  .same  length,  the  e.\tra  metal 


.■«^: 


('AN'TIM'.VKR    URHXiKS. 


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l»K   PONtlJiLTS. 


beiuj^  refiuiied  mainly  for  stiiTt'iiiiig  coriiiiii  truss  members  to 
resist  erection  stresses.  Of  course,  if  false  woik  be  used  for 
the  suspended  si)!in,  tlie  eight  i)er  cent  excess  will  not  l)e 
iuhled. 

The  curves  of  percentage's  are  based  on  two  assumptions, 
viz.:  first,  the  panels  througliout  the  entire  .structure  are  of 
ecjual  length,  and,  second,  the  lengths  of  the  cautilever-arms 
and  anchor-arms  arcthe  same.  The  first  assumption  is  nearly 
always  correct,  for  there  is  no  advantage  to  be  gained  l)y 
var^-ing  the  panel  lengths  in  tlie  various  portions  of  tlie 
bridge.  If  the  leugtlis  of  cantilever  and  anchor  arms  are  un- 
equal, the  average  weight  of  metal  obtained  for  the  latter  by 
use  of  th(!  curve  will  liave  to  bo  corrected  by  the  formula 

T 

where  T'  is  the  correct,  final  weight  of  truss  and  lateral 
melal  in  the  anchor-arm,  T  is  the  weiglit  of  same  found  by 
the  percentage  curve,  and  r  is  tlie  ratio  of  lengtii  of  cantilever- 
arm  to  that  of  anchor-arm. 

It  should  be  oi)served  that,  in  applying  the  percentage 
curves  to  structures  having  subdivided  panels  like  lliose  of 
tlic  Petit  truss,  the  niain  or  double  panel  is  to  be  \isc(i  as  the 
basis  of  calculation. 

The  me'hod  of  ai)plying  the  percentage  ("'rvesisas  follows: 
Let  us  tuUe  any  opening  and  assume  tiiat  tlie.  ,  .ue  six  panels 
in  eacli  cantilever-arm,  and  that  the  weight  per  foot  of  truss 
and  lateral  metal  in  tlie  suspended  .'■pan  i.«  /f-  the  n-inel  length 
being  ;),  aiid  ;>//>  =  W.  It  is  to  be  o'  i,.'  ti  .  this  method 
is  applicj'.ble  for  any  proportitMiate  li    ,,  u  (»f  suspemkd  span. 

The  weight  of  metal  in  the  lloor  system,  being  independent 
of  the  span  length  and  simply  a  function  of  .le  panel  length 
and  of  the  distance  belv.een  liUsses,  is  not  oousiderec!  in  tiu; 
investigation,  but  is,  of  course,  to  be  added  when  figuring  the 
total  weight  of  metal  in  the  structure. 

The  weight  of  truss  and  lateral  metal  in  the  cantilever-arm 
will  be 

\.2W-\-  \AW]-\MW  I  '2.0]F+3.4Tr-f  3.0Tr-11.65TK. 


CANTI!  KVKIl    HftlDOES. 


n 


The  weii,dit  of  metal  in  the  piiiiel  over  the  pier  is,  iiccordiiig 
to  the  directions  on  the  diagnun, 

1.8  X  •.i.OW=  5.4  U'. 

Let  us  assume  that  there  are  only  five  panels  in  the  anchor 
arm,  then  the  trial  weight  7' will  he 

Q.irtW't  i.7.-)}r-f  2.nK+  2.r)ir+  :].oif=  lo.ioir. 

Suhstittiting  in  the  formula  gives 


^„^1.L  10  1^3  G^^jj^j 

2  i>  \ 


W. 


It  will  he  seen  from  tlu-se  calculations  that  the  full  iiercent- 
ages  given* for  till' I'lui  jjiiiici  points  of  cantilever  and  anchor 
arms  are  to  be  used,  al'hou!,di  in  reality  there  is  hut  a  half 
l>anel  lengtii  for  eacii  point.  This  is  ctiuscd  by  the  heavy 
details  reipnred  at  these  points  for  aiijustment  and  anchorage. 
All  erection  metal  at  liie  end  of  a  suspended  span  is  a-ssunud 
to  belong  to  the  cantilcvci-  arm. 

Should  in  any  case  the  panel  lengths  be  unecjuul  in  dill'erent 
portions  of  the  structure,  it  will  be  a  simple  matter  to  use  the 
curves  hy  finding  avcriiLre  weights  per  foot  for  two  assumed 
cases  of  equal  panel  leiiLrths,  one  makiiiij  iIk-  arm  greater  and 
tiie  other  makinsi:  it  less  in  length  than  it  aclualiy  is,  and 
interpolating  properly  between  the  results  tor  tin;  retpiired 
average  weight  ))i'r  foot  for  the  arm. 

The  total  weight  of  nietai  in  tin;  two  anchorages  of  any 
three-span  cantilever  bridge  can  be  taken  '\i  five  jK'r  cent  of 
the  grand  total  weight  of  metal  in  the  said  three  spans,  and 
the  weight  of  nu'tal  in  the  pedestals  on  nniin  piers  at  four  per 
cent  of  same.  Of  course,  conditions  va.'-y  f<n- ditferent  ca.ses, 
nevertheless  these  juTcentages  will  give  results  sulhcicntly 
close  for  all  practical  p\n  poses. 

If  the  bridge  be  so  long  as  to  require  an  anchor-span,  its 
weight  of  trtiss  and  lateral  metal  per  lineal  foot  Avill  ])o  about 
3,25^/',  irrespective,  strange  to  .say,  of  the  length  of  said 
anchor  span,  w  being  the  Aveight  per  foot  of  the  trusses  and 
laterals  in  a  suspended  span,  whost"  length  is  three  eighths  of 


Y4 


1)E    I'ONTIIUS. 


Ilie  mniu  opening.  The  explanution  of  tliis  irs  that  the  wei/j^ht 
per  foot  of  the  chords,  tliough  intlepeiident  of  the  upward 
l)eii(liiig  moment,  inere.-ises  i^roportionjitely  to  tlie  downwaid 
bending  monu-nt  with  the  length  of  spun  ;  winle  tiie  weight 
per  fool  of  tiie  w'(^l),  in  so  far  as  it  is  jilTeelcd  liy  tlie  siiears 
from  exterior  loading,  the  rnling  factor  in  determining  tlie 
sections  of  web  members,  varies  inveisel}-  as  llie  span  length. 

If  the  length  of  anchor-span  he  very  short,  say  materially 
less  than  one  half  of  t  main  opening,  liie  Aveigiit  per  foot 
for  trusses  and  laterals  .>ill  have  to  be  increased  to  'iJnr,  not 
withstanding  the  fact  that  the  entire  top  chords  nniy  then  l)e 
built  of  eye-bars;  but  siirh  short  spans  would  probably  be 
barred  out  by  consideration  for  navigation  interests. 

The  percentage  curves  of  Plate  X  will  not  bear  a  rigid 
criticism,  in  that  they  nuike  the  weight  of  njctal  dejieud  ujton 
th(!  number  of  panels.  It  is  j>resu|>posed,  however,  tiiat  tiie 
panel  length  adopted  i.n  the  most  appropriate  one  for  the 
bridge;  and  the  curves  will  be  found  ((uile  accurate  wliencver 
llie  pniper  jianel  length  is  used.  With  long  jianels  tlie  weight 
of  nietal  pt^r  lineal  foot  found  by  tiie  curves  for  cantilever  and 
anchor  arms  is  h"^s  tlian  that  found  therebj-  for  short  panels. 
This  is  as  it  should  be,  but  to  a  limited  extent  only  ;  for  it 
can  b(!  found  by  trial  that  an  abnormally  siiorl  or  abin)rmally 
long  panel  length  will  give  results  too  gnat  or  loo  small  wlien 
checked  by  C()m|tutatioiis  of  weights  made  from  actual 
designs. 

'i'iiese  per(;enlage  curves  enabled  Mr.  lledrick  to  solve  reatl 
ily  the  n<  \t  problem,  viz.,  given  the  total  distance  iH-twe.ii 
centres  of  anchorages  and  c<i/7<'  bldiirlie  as  lo  the  location  of 
the  main  piers,  to  detcrniiue  tlie  lenglh  of  each  anchor-arm 
wliicli  will  make  tiie  total  v/eigiit  of  metal  in  the  structure  a 
minimum.  He  f.>und  tiiis  length  to  be  two  tenths  of  th(!  tt)tal 
di.stance  between  the  am  b  uages. 

It  must  not  be  forgotten  that  for  every  dollar  saved  by  re- 
ducing the  toinl  weight  of  metal  through  the  shortening  of 
tlie  anchor-arms,  it  will  be  necessary  to  spend  about  twenty 
cent.s  for  extra  concrett' in  the  anchorages.  On  t ids  account, 
for  tlie  conditions  assiinud,  tlie  truly  economic  length  of  <  :i<  ii 


CAN'TILKVKIJ    JUUlXiKS. 


rs 


;tli. 
ally 
foot 
not- 
II  lie 
be 


(inehoruim  of  u  tluct'-spiiri  cHiililcvcr  will  gciiorally  be  a  littlt; 
^renter  lluiii  Iwent.y  per  cent  «)f  the  total  (lisljuicc  between 
(Seniles  of  anelKM-iiircs. 

When,  liowever,  the  problem  is  to  determine  the  economic 
lenifth  of  anchor-arm  for  a  lixeil  distance  between  main 
piers,  the  result  will  be  (piite  (lifTereiil  ,  ])C(rause,  within 
reasonable  limits,  the  shorter  Hk;  anclior-arm  tiic  smaller  will 
be  its  total  weight  of  metal,  iiiid  because  tresth;  approach  is 
much  less  expensive  than  anchor-arm.  It  would  not,  for 
cvidciil  reasons,  l>e  advisable  to  make  the  length  of  anchor- 
arm  less  than  twenty  per  cent  of  that  of  the  main  ()i)eiung,  or 
say  lifteen  jiir  cent  of  the  total  distance  between  centres  of 
ancliorages.  With  this  length  there  would  probably  be  no 
reversion  of  stress  in  the  cliords  of  the  anchor-ium,  even 
when  impact  is  considered.  Generally,  though,  the  ajtpear- 
aiKie  of  the  structure  will  be  improved  by  using  longer  ani;hor- 
arms  than  the  infe'ior  limit  just  suggested. 

In  respect  t  >  the  best  proportionate  length  of  anchor- 
spans,  the  latter  weigh  so  much  per  lineal  foot  for  all  cases 
liiat  the  shorter  they  are  made  the  greater  the  economy  ,  but, 
as  before  stated,  it  i.i  improbable  that  navigation  interests 
would  ever  jiennit  of  their  being  made  shorter  than  one  half 
of  tlie  main  openings. 

In  respect  to  his  lifth  and  last  prol)lem,  Mr.  Iledrick 
obtained  the  following  restdls  . 

The  total  weight  of  metal  in  a  threo-span  caittilever  railroad 
bridge,  tloor  system  incluiied,  is  to  tiie  total  weight  of  metal 
in  a  simple-span  Inidge  of  thnvocpial  openings,  for  which  false 
work  is  to  be  used  throughout,  ;is  unity  is  to  0.0.  The  cor- 
responding ratio  for  the  case  of  the  i-eatre  span,  elected  with- 
out false  work,  is  unity  to  0  (U. 

For  a  very  long  bri<lge,  eomp*\sv«l  of  a  suc^ceasion  of  canti- 
levers an<l  anchor-spans  which  are  one  half  as  longasthemtiin 
openings,  and  which  has  a  suspended  span  resting  on  cacli 
extreme  pier,  the  ratio  of  wt  ighi  of  metal  to  that  in  a  corres- 
|»<>nding  liridge  of  equal,  sim|ilc  spimsand  the  .sanu!  mimber  of 
piers,  the  spans  being  erected  on  false*  work,  is  its  imiiy  to  0.75. 
For  Ihe  case  of  allern;ile  sinip'e  spans  erected  without   fal.se 


7« 


i\K    I'ONTIfU'S. 


work,  the  ratio  would  bo  as  unity  to  0.8.  Tlii'so  results  were 
obtained  by  assuming  average  probable  conditions  ;  but  the 
longer  the  sinii)le  spans  and  the  greater  the  total  lenglii  of 
structure,  the  less  will  be  the  variation  in  weights  of  cantilever 
and  simple-span  l)ridges,  althougii  it  would  reipiire  very  long 
spans  and  a  great  total  length  of  structure  to  change  material- 
ly the  rat. OS  found. 

It  is  therefore  evident  that,  when  economy  in  first  cost  is 
considered,  as  it  always  ought  to  be,  there  will  seldom,  if  ever, 
be  any  need  for  considering  the  adoption  of  cantilever  bridges 
with  anchor  spans,  because  structures  wiih  simple  spans  are 
l»oth  cheaper  and  better.  It  is  also  evident  that  in  many  eases 
it  is  advisable,  from  (lonsidcrations  of  botji  rigidity  and 
economy,  to  adopt  a  bridge  consisting  of  three  simple  spans, 
with  the  middle  one  cantilevered  from  the  others,  rather  than 
the  ordinary  three-span  cantilever  bridge.  When  each  of  the 
side  si)ans  is  as  short  as  one-half  of  the  middle  span,  or  even 
shorter,  there  will  be  no  dillieulty  experienced  in  the  erection, 
and  no  great  ])rovision  will  be  re<iuired  for  holding  down  the 
outer  ends  of  the  side  spans  during  erection.  Of  cour.-e,  the 
nearer  to  eciuality  that  the  three  spun  lengths  are  made,  the 
greater  will  be  the  economy  of  metal,  but  a  wide  divergence  in 
these  lengths  will  not  necessitate  any  such  increa.se  iu  weight 
as  to  alter  the  preceding  cf)nelusi()n  regarding  the  great  econ- 
omy of  simple-span  bridges  over  ordinary  cantilever  structiu'es. 

The  question  sometimes  arises  .as  to  how  the  total  weight  of 
metal  in  a  three-span  cantilever  bridge  varies  with  the  length 
of  the  main  opening.  If  the  lengths  of  the  anchor-arms  vary 
l)roportioiiately  to  the  main  opening,  the  increa.se  or  decrease 
in  the  total  weight  of  mc  lal  in  the  structure  will  vary  ahoul 
twice  as  rapidly  as  the  increase  or  decreas*;  in  length.  For 
instance,  if  the  main  opening  and  total  h  ngth  of  bridge  be 
increased  ten  per  cent,  the  total  weight  of  metal  in  the  entire 
structure  will  be  increased  twenty  jx'r  cent.  This  rule,  which 
is  merely  an  appro.xiinatioii,  will  apply  fairly  well  for  changes 
not  exceeding  twenty  per  cent  and  for  spans  of  uiedium 
length.  For  greater  changes  tin;  ratio  of  increa.se  or  decrease 
gradually  augments,  and  for  very  long  spans  it  is  slightly 
greater  than  two,  while  for  very  short  ones  it  is  slightly  less. 


CANTILKVKU    ItIM  l)(i  DS, 


77 


In  icspccl  to  iho  relations  wliicli  sliould  fxi^l  between  lenglli 
of  main  opening,  peipendieulur  disluncx'  between  ceii'  ..1 
pbmes  of  trusses,  and  the  various  lni>s  depliis,  the  autiior's 
practiee  is  to  nialie  the  least  disiancc  iH'tW(  en  parallel  trusses 
one  tw<;nt}'-sevenlh  of  the  main  openini;:  the  least  distance 
between  axes  of  vertical  posts  over  niairi  piers,  wlieii  the 
trusses  converge  towards  the  suspended  span,  one  Iwent^-iifth 
of  the  said  opening;  the  truss  dcptli  for  the  suspended  span, 
when  the  cliords  are  parallel,  from  one  fifth  to  one  sixth,  or 
for  very  long  spans  even  one  seventh,  of  the  s]mn;  ai;d  the 
height  of  tlie  vertical  posts  over  main  i)i(!rs  not  to  exceed  foui', 
or  preferably  three  and  a  half,  times  the  lu-rpendicular  dis- 
tance between  their  axes.  For  through  cantilever  bridges 
llie  author  generally  makes  the  height  of  these  posts  about 
lifteen  per  cent  of  the  length  of  the  main  opening. 

For  tlie  sake  of  appearance  the  centres  of  the  toi)-chord  i)ins 
in  cantilever-arms  are  i)lac(d  on  arcs  of  parabolas,  the  vertices 
of  which  are  located  at  the  hips  of  the  suspended  span;  and 
the  anchor-arms  are  laid  out  to  the  same  curve,  beginning  at 
the  toi>s  of  the  ])osts  over  the  main  piers. 

In  concluding  this  chapter,  u  ciieck  on  the  correctness  of 
percentage  curves  for  weights  of  cantilevers  will  be  given  by 
applying  the  (Mirves  to  the  published  estimated  weights  of 
metal  in  tlu;  various  members  of  the  longest  cantilever  bridge 
that  has  ever  yet  been  designed  in  detail,  viz.,  the  proposed 
2'iOI)-ft.  span  (meas»u-ed  between  ceidres  of  main  piers)  for 
the  North  Jtivcr  Bridge  at  New  York  City.  This  proposed 
.structure  was  tiesigned  by  the  Union  Hridgc;  Company. 

Tiie  total  weight  of  metal  in  trusses  and  laterals  of  the  730- 
ft.  susi)en(led  span  is  10,400,1)00  lbs.  'I'lic  trusses,  which  are 
of  the  I'etit  type,  are  divided  into  six  nuiiu  pancils  of  1'.20  ft. 
eacli;conse(iuently  the  panel  weight  is  10,-100,000  :  (i^l.TJW.OOO 
lbs.  In  the  cantilever  arm  theie  are  six  and  five  eighths  nuiin 
panels;  conseijuently  the  weight  of  trusses  ami  laterals  there- 
for will  be 


i.20W-]-\A0W~\   l.fi-jVr  f  2.00  )Kj- 3.40  W^  t   3.00  IK 
i-  i  X  3.«0  W  =  i;}.!»0  W  '-  34,01)0.000  Ib.s. 


78 


i)K  PONTinrs. 


Tj'.u'h  !iricli()r-arni  is  840  ll.  li>iiir,  niid  is  divided  into  seven 
doiihle  piiiicls,  mid  tiieic  aie  seven  and  (ive-ei,L;iitlis  loads  to  be 
considfiod;  eonse(iiicntly  the  wciglil  of  trusses  und  laterals 
llierefor  will  be 

0.75  ]V  \  1.75  W  \  2.10  W-j-  2.50  JK-j- 3.00  TK-|- 8.75  \V-\    1.75  W 
-\-  I  X  5.(15  ]V  z^  22.13  W  -  :j^;551,000  Ihs. 

'I'liis  weight  niiist  l)e  rediicod,  ouiii.iT  to  the  fact,  tliiit  Ihe 
length  of  tlie  cantilever  arm  is  only  si.\  sevenths  of  tliai  i>\' 
Ihe  anchoi-arni,  making  /•  —  0.y57. 

7"  =  -(1  +  ;•)  =  '-''^'''•"*"\l.t?57)  ^  35,009,000  lbs. 

The  total  weight  by  the  curves  for  the  two  cantilevc  i  and 
unciiur-arms  is  therefore 

2(24,090.000  f  35,009,000)  =  119,308,000  ll)s. 

The  total  weight  of  metal  given  in  the  i)ui)lished  cslimatc 
for  trusses  and  laterals  for  the  two  cantilever  and  anchor  arms, 
after  deducting  11,500,000  lbs.  for  weight  of  metal  in  liie 
anchorages  and  ignoring  the  allowance  for  sundries  (which 
was  prol)ably  put  in  for  prudential  reasons),  is  119,700,000 
lbs.,  making  the  dillerence  802,000  ibs. ,  or  about  one  (piarler 
of  one  per  cent. 

This  is  an  extremely  accurate  check,  and  proves  that  the 
curves  are  reliable  ;  nevertheless  the  author  would  not  guar- 
antee them  to  give  any  such  close  coincidence  for  all  cases. 

Since  tiiese  pages  went  to  press  Ihe  author  has  been  engaged 
on  the  making  of  a  jireliminary  design  with  a  delailcd  oil 
male  of  weight  of  metal  for  a  proposed  double-lrac;k  railway 
and  Inghway  cantilever  bridge,  with  a  central  opening  of 
1,(500  ft.,  to  cross  the  St.  Lnwremo  Hiver  near  Queb(!c, 
Canatla.  The  result  of  the  estimate  as  far  as  it  has  been  car- 
ried gives  another  excellent  check  on  tiie  accuracy  of  oue  of 
the  curves  ;  as  the  error  for  Ihe  (.antiiever  arms  is  only  one- 
eiglith  of  one  per  cent.  The  anchor  arms  have  not  yet  been 
(Jt;  tailed. 


CHAPTER  VI. 


AKCHE8. 


TnK.  tirch  is  II  nitlier  iituoinnioii  type  of  strnrliire  in  Anier- 
icu,  liccHiisc  the  conditions  wliicli  iiiukc  il  fconoiuirai  aic 
unusual.  For  docp  gorges  with  rocliy  sides,  or  lor  shallow 
slreiinis  witli  lock  boUoin  and  natural  abuliucnts,  arclies  are 
eminently  jjiopcr  and  cconoiuical.  But  when  a  steel  holloni 
chord  is  needed  to  take  up  the  liirusl  between  springing 
points,  all  the  economy  of  the  arch  vanishes. 

The  advantages  of  tlie  arch  are  a  possible  economy  of  metal 
and  an  ieslhetic  api)earance,  while  its  disadvantages  are  a  lack 
of  rigidity  and,  for  most  types,  an  uncertainly  concerning  the 
nuiximum  stresses  in  the  members. 

Arches  are  sometimes  used  for  large  train-sheds,  in  which 
their  architectural  ellect  is  certainly  very  flue,  but  tliey  require 
about  twice  as  much  metal  as  docaiitilevered  trusses  supported 
on  colinnns;  conscniuently  they  can  be  adopted  only  when 
appearance  is  an  extremely  important  factor  in  the  design. 

When  bridge  foundations  have  to  be  built  on  piles  or  on  any 
oilier  material  that  is  liable  Ui  slight  .'settlement,  or  when  the 
abutments  roidd  po.ssibly  move  laterally  even  a  mere  trille,  it 
is  nut  [uoper  to  adopt  an  arch  superslruelure;  for  any  seltle- 
nuMit  or  any  motion  whatsoever  in  eitlier  piers  or  abutments 
would  up.«et  the  conditions  assumed  for  the  compulations,  and 
thus  cau.se  to  be  increased  to  an  uncertain  amount  some  of  the 
stresses  for  which  the  superstructure  was  proportioned.  This 
criticism  does  not  apply  to  the  three-hinge  1  arch,  but  even  this 
design  reiju ires  good,  solid  abutments  ann  firm  foundations  for 
piers. 

Arches  cau  be  erected  on  false  work,  by  cantilevering,  or  by 

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Photographic 

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23  WEST  MAIN  STREET 

WEBSTER,  N.Y.  14580 

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80 


DK    PONTIBLS. 


biiiUUiig  vertically  the  two  halves  aud  lowering  them  by  cables 
till  they  meet  at  ti»e  centre.  Whichever  of  these  methods  is 
the  easiest  and  cheapest  is  the  one  to  adopt. 

A  very  easily  erected  arch  is  shown  in  Fig  4.  The  pieces 
marked  AB  are  temporary,  and  are  to  be  used  only  during 
erection.     They  can  be  made  of  timber,  so  as  to  be  lemoved 


Fio.  4. 

readily  after  the  arch  is  coupled  at  mid-span,  or  may  be  of 
steel,  and  be  left  in  as  idle  members,  solely  for  the  sake  of 
appearance. 

It  will  be  seen  from  the  diagram  that  the  structure  is  a 
cantilever  during  erection,  and  afterwards  consists  of  an  arch 
span  and  two  simple  spans.  This  type  of  bridge  probably 
requires  a  little  moie  metal  than  would  an  ordinary  segmental 
arch  with  trestle-approaches,  and  possibly  is  not  quite  as  rigid 
as  the  latter,  but  the  saving  of  cost  in  erection  will  fully  oflfset 
these  disadvantages. 

With  three  hinged  arches  there  is  no  ambiguity  whatsoever 
in  the  determination  of  stresses,  but  in  all  other  cases  there  is. 

There  are  four  cases  all  told,  viz.: 

1.  Arch  without  any  hinges. 

2.  Arch  with  one  hinge  (at  crown). 

3.  Arch  with  two  hinges  (at  abutments). 

4.  Arch  with  three  hinges  (a   crown  and  abutments). 
These  four  cases  can  be  reduced  to  three,  because  there  is 

no  good  reas<Mi  for  ever  building  an  arch  dxed  at  the  abut- 
ments and  hinged  at  the  crown. 
Jii  (!ase  No.  4  there  are  no  temperature  stresses,  but  in  all  of 


ARCHES. 


81 


the  otlier  cmsos  there  are  ;  and  I  lie}'  must  always  receive  due 
consideralion  in  proportioning  the  members. 

All  things  considered,  the  uiulior  prefers  to  adopt  ilie  three- 
hinged  arch  for  ruilroiid  bridges,  because  ilie  stresses  can  be 
determined  as  accurately  as  can  those  of  an  ordinary  truss 
bridge,  and  because  of  llie  absence  of  temperature  stresses  ;  at 
tlie  same  time  it  must  be  admitted  that  an  arch  witliout  hinges 
is  more  rigid  than  one  with  hinges,  and  tliat,  theoretically,  it 
is  more  <;cononiical  of  metal. 

For  Idghway  bridges,  in  which  the  assumed  live  loads  will 
seldom,  if  ever,  be  realized,  it  would  be  best,  all  things  con- 
sidered, to  ado|>t  the  arch  without  hinges,  so  as  to  obtain  tlie 
greatest  po.ssible  rigidity,  even  at  the  expense  of  certainty  in 
computing  stresses. 

For  arched  train -sheds,  the  two-hinged  arch  of  crescent 
shape  will  generally  be  foiuul  the  most  satisfactory. 

Winle  the  author  was  engaged  on  the  i)reparation  of  this 
ciiapter  he  received  a  copy  of  Prof.  Malverd  A.  Howe's  new 
book,  entitled  "  A  Treatise  on  Arches."  This  work,  which  is 
entirely  mathematical  in  character,  is  certainly  the  most  com- 
plete book  on  arciies  that  has  ever  been  written,  and  appears 
lo  cover  the  entire  subject  of  stres.ses  in  arches  of  all  kinds  in 
a  most  satisfactory  manner,  altiiough,  of  course,  the  author 
cannot  vouch  for  the  correctness  of  Prof.  Howe's  figures 
without  checking  the  matliematics  from  start  to  linish,  a  task 
which  lie  feels  is  too  great  for  both  his  spare  time  and  his 
advancing  years.  It  is  probable,  tliough,  that  the  autlior  will 
have  the  book  checked  some  time  by  one  of  his  assistant  engi- 
neers, in  ca.se  tliat  he  has  t  >  make  another  design  for  an  arch. 
Meanwhile  lie  is  satistied  to  assume  that  all  of  Mie  mathemati- 
cal work  is  correct,  because  of  Prof.  Howe's  established  repu- 
tation as  both  a  mathematician  and  an  engineer.  Prof.  ^lowe 
has  tabulated  the  res\ilts  of  his  compulations  in  a  very  con- 
venient form,  so  that  his  forniulw  can  readily  be  applied  in 
designing,  especially  for  preliii'inary  designs  and  estimates. 
In  .spite  of  its  discouragingly  mathematical  appearaiute.  Prof. 
Howe's  book  promises  to  prove  of  great  practical  value  to 
designers  in  utruclurul  st(cl  ;  :in(i  its  author  is  certainly  to  be 


82 


1)E   PONTIRUS. 


commended  for  the  immense  cllori  lie  liiis  put  fortli  in  accom- 
plisliiiig  for  the  eugiueeriug  profession  such  a  luoorioua  piece 
of  work. 

Prof.  Howe  finds  the  reliiiive  weiglits  of  metal  in  a  416'  arch 
with  a  67'  rise,  for  Cases  Nos.  1,  3,  and  4,  to  be  as  follows  ; 

Case  No.  1,  no  hinges 1.00 

Case  No.  3,  two  hinges 1.21 

Case  No.  4,  three  hinges. 1.30 


The  author  is  of  the  opinion  that,  if  he  were  to  make  three 
such  designs  for  coinpaiison,  there  would  not  be  such  great 
diflfe'-ences  in  the  weights,  because  constructive  reasons  will 
cause  the  designer  to  use  only  a  few  diireront  sectional  areaa 
in  the  chords  of  an  arch,  while  Prof.  Howe's  students,  who, 
as  he  stales,  made  the  calculations  from  which  the  tabulated 
ratios  were  determined,  probably  proportioned  the  section  of 
each  panel  length  of  each  chord  for  the  greatest  stress  to  which 
it  could  1)6  subjected.  This  would  be  eminently  proper  in 
making  such  a  comparison;  but  the  resul'sof  the  computatiouH 
would  not  agree  with  similar  results  obtained  by  a  bridge 
specialist. 

It  is  ditiicult  to  nwike  a  proper  comparison  in  re8j)ect  to 
economy  lie. ween  arched  and  simple  truss  bridges,  owing  to 
the  fact  that  the  piers  differ  for  the  two  cases;  but  a  fair  one 
can  be  obtained  by  assuming  that  steel  braced  piers  are  used 
to  siipport  the  deck  span. 

The  author  has  had  occasion  lately  to  design  in  complete 
detail  for  a  British  Columbia  railroad  a  260- ft.  arch  bridge, 
shown  in  Fig.  5,  h.'.viug  a  rise  on  the  centre  line  of  59  ft.,  and 
to  compute  the  exact  weight  of  metal  in  same.  For  the  sake 
of  comparison,  he  has  since  designed  according  to  the  same 
specifications  a  260  ft.  deck-span,  having  a  truss  depth  of  35 
ft.,  resting  on  steel  braced  towers  36  ft.  high.  The  total 
weight  of  ntetal  tor  the  arch  design  is  2,111  pounds  per  lineal 
foot,  and  that  for  the  truss  design,  including  the  towers,  is 
2,542  pounds  per  lineal  foot,  showing  a  saving  of  about  twenty 
per  cent  in  favor  of  the  arch. 


ARCHES. 


83 


As  for  the  relative  rigidities  of  liiese  two  structures,  tlicre  is 
very  little  doubt  that  a  con)|)urisou  of  the  tiuislicd  bridges 
under  load  would  result  ia  favor  of  the  simple  spun. 

la  making  the  prelimluary  study  for  the  arch  bridge  herein 


Fio.  5. 


referred  to,  there  was  prepared  a  cimiparative  design  for  a 
three  hinged  arch,  in  which  each  half  oi  each  arch  consists  of 
a  lenticular  truss  as  shown  in  Fig.  6. 
Contrary  to  the  author's  surmise,  this  design  did  not  prove 


..--A 


tAiR>ra9!*»m:Mi 


Fio.  6. 


to  be  any  more  economical  than  that  with  the  circular  arch, 
t,hfi  toUl  weights  of  metal  in  thu  two  structurea  being  ahinn 


84 


DE   PONTIBUS. 


exactly  the  same.  Tlie  circular  arch  was,  therefore,  adopted 
on  account  of  its  superior  appearance. 

Couceruiiig  the  rehuions  between  the  principal  dimensions 
for  arch  bridges  of  various  types  but  little  can  1x5  said,  for  tiie 
reason  that  but  little  is  known,  because  of  the  sci'vrcity  of  such 
bridges  in  ihis  country.  In  most  cases  the  length  of  span  a*:d 
the  rise  are  determined  by  the  existiig  conditions  at  the  cross- 
ing. For  any  given  span,  the  greater  the  rise  the  less  tlie 
ellect  of  uniform  load  stresses,  but  the  greater  the  elfect  of 
partial  load  stresses,  and  vke  verna.  Again,  for  any  given 
span  and  rise,  the  -\rch  depth  does  not  affect  the  uniform  load 
stresses  materially,  while  it  does  so  affect  the  partial  load 
stresses;  and  as  the  latter  are  inferior  in  importr.nce  to  the 
former,  it  results  that  the  depth  of  an  arch  for  economy  of 
material  will  be  very  much  less  than  the  best  depth  for  an 
ordinary  truss  of  the  same  span.  The  arch  depth,  too,  will 
depend  upon  whether  the  arch  has  fixed  ends  and  continuous 
crown,  hinged  ends  and  continuous  crown,  or  hinged  ends  and 
hinged  crown.  For  the  Urst  type,  a  varying  depth  increjising 
from  the  centre  to  the  ends  is  econojnic;  for  the  second,  a 
varying  depth  increasing  from  ends  to  centre  is  l>est;  while 
for  the  third,  a  constant  depth  from  end  to  end  seems  prefer- 
able. Again,  the  arch  depth  will  depend  considerably  upon 
the  style  of  web,  i.e.,  whether  it  be  plate,  open-riveted,  or 
pin-connected.  The  best  depth  or  depths  to  adopt  for  any 
case  should  be  given  a  special  study,  in  miikiug  which  Chapter 
VI  of  Prof.  Howe's  book  will  be  found  of  great  assistance. 

In  respect  to  the  style  of  curve  to  adopt,  whether  circular, 
parabolic,  or  elliptical,  the  author's  preference  would  generally 
be  for  the  circular  on  account  of  its  simplicity,  although  the 
parabolic  might  theoretically  give  better  results. 

In  the  author's  opinion,  a  plate-girder  arch  should  be  made 
without  hinj-'is,  an  open-webbed  riveted  arch  either  with  or 
without  hinges,  and  a  pin-connected  arch  with  hinges.  In  the 
latter  case,  it  is  only  the  web  members  that  should  be  pin- 
connected,  for  the  chord  members  should  be  riveted  up  and 
fully  spliced  from  end  to  end.  There  slioitld  be  only  a 
jingle  system  of  cancellation  used  in  webs  of  arches,  so  as  to 


ARCHES. 


86 


Avoid  as  mucb  as  possible  ninbiguity  in  tbe  stress  distribution. 
Riveled  connections  arc  preferable  to  pin  connections  for  the 
diiigonals  on  acconnt  of  rigidity,  but  are  more  expensive  for 
erection. 

Hard  and  fast  rules  for  the  miuinmm  spacing  of  outer  arches 
of  bridges  for  various  spans  and  rises  cannot  well  be  given. 
The  narrower  tbe  structure  within  reasonable  limits  the  less 
the  cost,  but  the  less  also  the  rigidity  and  the  lateral  resistance 
to  overturning  from  wind-pressure.  In  the  a60-ft.  span  herein 
rt  rred  to,  the  author  made  the  distance  l)etween  c«mtral  planes 
of  a  hes  twenty-two  feet,  which  was  as  small  a  distance  as  he 
dared  to  adopt,  notwithstanding  the  fact  that  economy  of  first 
cost  was  an  important  factor  in  the  design.  An  approximate 
rule  to  work  by  might  be  to  make  the  perpendicular  distance 
between  outer  arches  not  less  than  one  third  of  the  height  from 
springing  point  to  grade. 

In  concluding  this  ciMipter,  the  author  desires  to  call  atten- 
tion to  the  fact  that  there  is  still  a  great  deal  to  be  learned 
about  the  designing  of  arches  ;  and  to  suggest  that  some  pro- 
fessor of  civil  engineering,  who  is  well  posted  on  bridge  de- 
signing and  who  has  time  to  spare,  could  spend  several  mouths 
to  the  great  advantage  of  the  engineering  profession  in  deter- 
mining the  proper  relations  of  span  length,  rise,  arch  depth, 
width  between  exterior  arches,  etc.,  for  the  variims  styles  of 
arch,  and  in  ascertaining  the  relative  econonnies  of  the  latter. 


CHAPTEK  Vn. 


TRESTLES  AND  VIADUCTS. 


But  little  need  be  said  in  this  chapter  concerning  the  design- 
iug  of  trestles  and  viaducts,  as  that  subject  is  fully  covered  in 
Chapters  XIV  and  XVI.  However,  as  the  latter  chai)lers  are 
BpecificHtioiis,  and  are  written  in  very  concise  form,  it  seems 
advisable  to  give  Ijcre  certain  cxphuiatious  of  the  reasons  for 
the  rules  and  directions  titoreiu  formulated,  even  at  tlie  risk 
of  repetition  of  a  few  matters. 

The  best  layout  for  a  trestle  or  viaduct  is  the  one  which  will 
make  the  cost  of  tho  structure  a  minimiun,  provided  that  the 
speciUcations  used  in  designing  will  insure  for  any  layout  tlie 
requisite  strength  and  rigidity. 

As  stated  in  Chapter  III,  the  greatest  economy  will  exist 
when  the  cost  of  the  bents  and  their  pedesUvls  is  equal  to  the 
cost  of  the  longitudinal  girders  and  longitudinal  bracing.  On 
this  account  it  is  advisable  to  make  the  t(  'ver  spans  shorter 
than  the  intermediate  spans,  taking  care,  however,  pot  to  have 
the  former  too  short  for  either  appearance  or  proper  resistance 
to  traction.  In  general,  tower  spans  should  vary  in  length 
from  twenty  to  thirty  feet,  although  for  very  low  structures  it 
may  son)etinies  Ije  advisable  to  go  a  few  feet  below  twenty. 
For  the  intermediate  spans  the  length  generally  varies  from 
thirty  to  sixty  feet ;  but  for  very  low  structures  with  heavy 
rolling  loads  the  economic  length  may  be  found  to  be  less  than 
thirty  feet,  in  which  case  it  will  be  perfectly  legitimate  to  re- 
duce the  span  length  to  suit  the  economic  conditions. 

The  reason  for  adopting  sixty  feet  for  the  superior  limit  is 
because  trestles  and  viaducts  are  nearly  always  erected  without 
falsework  by  starting  erection  at  one  end  of  the  structure  and 
dropping  the  members  down  by  means  of  an  overhanging 

86 


TUKSTLES   AND   VIADUCTS. 


»t 


traveller  running  on  top  of  tli(!  crectdl  portion  of  the  work. 
With  tower  spiins  of  thirty  feet  and  inlermediiite  spjins  of  sixty 
feet,  the  tniveller  will  have  to  reach  out  ninety  feet  to  erect  n 
tower,  which  is  about  the  extreme  jiracticable  limit.  However, 
.should  it  be  nece.s.sary  to  use  more  or  less  falsework,  longer 
spans  than  8i.\ty  feel  would  pn)bai)ly  be  economic. 

The  niost  economic  layout  for  a  highway  viu'luct  with 
wooden  joists  is  alternate  towers  and  spans  that  consist  entirely 
of  joists,  the  limiting  lengths  of  span  being  about  twent}' 
feet  for  the  towers  and  twenty-four  feet  for  intermediates, 
which  latter  length  is  the  greatest  span  u  ed  in  general  prac- 
tice for  4"  X  16"  wooden  joists.  It  is  not  leu;itimale  in  such  a 
design  to  rely  on  the  wooden  joisls  of  I  he  tower  spans  to  act 
as  a  part  of  the  longitudinal  tower  bracing. 

In  railroad  trestles  the  longitudinal  girders  should  abut 
against  and  rivet  into  the  webs  of  the  columns,  the  latter  being 
bent  just  below  the  longitudinal  girders  when  the  legs  are 
battered.  Tiie  author  has  lately  adopted  this  detail  in  some 
trestle  designs  for  a  British  Columbia  railroad,  and  has  found 
it  to  be  very  satisfactory.  For  double  track  structures,  the 
columns  at  tops  are  to  be  spaced  a  distance  equal  to  the  sum 
of  the  perpendicular  distance  between  the  longitudinal  girders 
of  one  track  and  that  between  centres  of  tracks,  and  the  legs 
may  be  made  vertical  up  to  a  limit  of  !d)out  twice  the  perpen- 
dicular distance  between  axes  of  opposite  columns. 

For  single-track  structures,  it  is  generally  best  to  space  the 
longitudinal  girders  and  tops  of  columns  ten  feet  centres,  al- 
though an  eight-foot  spjicing  is  legitimate.  The  former  spac- 
ing gives  greater  rigidity  to  the  structure,  but  necessitates  the 
use  of  deeper  timber  ties.  By  using  very  deep  ties  a  greater 
girder  spacing  may  be  adoptcsd;  but  this  is  not  necessary,  un- 
less very  long  intermediate  spans  erected  on  falsework  be  em- 
ployed. 

It  is  not  worth  while  to  use  a  batter  for  columns  less  than 
one  and  a  half  inches  to  the  foot,  and  it  is  never  economical 
lo  use  one  greater  than  three  inches  to  the  foot.  Tlie  smaller 
the  batter  the  less  the  total  weight  of  transver.^e  bracing,  but 
the  greater  the  tension  stres.ses  on  the  columns.     As  a  rule,  it 


ss 


1)R   POM  I  HUB. 


is  best  to  keep  these  teii>^ioii  sirt'sscs  low  or  even  to  make  tliein 
noil  existent;  but  in  iiigli  iresllcH  it  becomes  necessHry  to  per- 
mit and  provide  for  tliem. 

It  is  wbeu  trestles  are  on  sliarp  curves  tliat  great  butters  must 
be  used,  in  order  to  provid<!  against  tlie  overturning  tendency 
of  the  combined  centrifugid  force  and  wind  load.  In  sucli 
cases  as  these  with  Idgli  trestles  it  becomes  necessary  to  divide 
up  the  transverse  bracing  of  tlio  lower  portion  of  the  tower  by 
placing  short  vertical  columns  in  the  middle  t>f  the  bents,  and 
liracing  longitudinally  between  the  vertical  columns  of  alter 
iiate  adjacent  bents. 

In  very  high  trestles,  especially  when  located  on  siiarp 
curves,  the  combinations  of  coitunn  stresses  for  live  load,  dead 
load,  traction  load,  centrifugal  load,  and  wind  load  run  ex- 
tremely high,  and  demuiiti  great  column  sections  ;  conse- 
quently in  such  cases  it  becomes  necessary  for  the  designer  to 
use  considerable  good  judgment  so  as  to  reduce  the  toUil  stress 
to  reasonable  limits.  For  instance,  the  traction  stresses  can 
be  cut  down  to  leca  than  one  half  b}^  riveting  tlie  longituditud 
girders  of  an  intermediate  span  to  the  towers  at  both  ends. 
This  reduces  the  thrust  of  train  owing  to  the  iiicreased  length 
of  structure  used  for  determining  tlie  etiuivaleiit  uniforni  load, 
and  fixes  the  tops  of  the  towers  so  as  to  make  a  point  of  con- 
tratlexure  at  mid-height,  tlius  reducing  the  lever-arm  and 
therefore  the  bending  moment  to  one  half. 

Again,  unless  the  grade  be  heavy,  it  is  often  legitimate  to 
assume  that  tiie  velocity  of  train  is  materially  lessened  by  the 
sliarp  curve  by  the  time  tliat  the  train  reaches  the  high  portion 
of  the  trestle;  and,  as  the  centrifugal  load  varies  as  the  square 
of  tlie  velocity,  the  stresses  from  this  load  will  be  greatly  re- 
duced by  the  assumption. 

Again,  the  prevailing  high  winds  and  the  centrifugal  loads 
may  act  against  each  other  instead  of  together,  and  the  com- 
bination may  be  lowered  in  amount  by  recognizing  this  fact. 

In  short,  the  designer  in  such  a  c.-ise  can  use  his  judgment 
to  great  advantage,  and  thus  stive  considerable  metal  that  is 
not  really  needed,  although  it  might  be  required  if  a  strict  ad- 
berence  to  the  .specifications  were  enforced. 


TRESTLES   ANH   VIADUCTS. 


80 


Till!  best  style  of  bniciiigfor  l)()th  Vac  longitudinal  ami  trans- 
verse  faces  of  the  towers  consists  of  stiiTdiagoniils,  each  formed 
of  four  angles  with  u  single  line  of  lacing,  all  of  stdd  diagonals 
being  riveted  to  the  columns  and  to  each  other  where  they  in- 
tersect by  means  of  plates,  and  no  horizontal  struts  being  used 
except  at  top  and  bottom  of  towers,  where  they  are  necessary 
to  make  the  bracing  a  complete  system.  The  panel-points  of 
the  longitudinal  bracing  should  coincide  with  those  of  tiie 
transverse  bracing,  although  near  the  top  of  the  tower  the 
panels  of  the  latter  may  be  divided  on  account  of  the  small 
distance  between  columns. 

In  cheap  structures,  expense  can  be  saved  by  making  the 
diagonals  of  the  sway-bracing  of  adjustable  rods,  and  putting 
in  horizontal  struts  at  the  panel  points,  which  struts  should 
always  be  riveted  at  their  ends  to  the  columns  ;  because  pin- 
connected  struts  do  not  stiffen  the  columns  sufficiently  to  war- 
rant the  figuring  of  the  latter  as  fixed  at  the  panel  points. 

When  adjustable  diagonals  are  adopted,  the  employment  of 
horizontal  struts  at  top  and  bottom  of  towers  on  all  four  faces 
is  even  more  imperative  than  it  is  when  stiff  diagonals  are 
used.  The  author  has  seen  trestles  without  such  bottom 
struts,  in  which  the  columns  have  been  moved  considerably 
out  of  place  by  the  rods  contracting  in  cold  weather  and 
drawing  the  column  feet  together.  Six  months  afterwards 
the  rods  elongated  and  hung  in  festoons,  so  were  prom|)tly 
tightene<l  up  by  the  bridge  insjwctors,  thus  putting  them  in 
good  (condition  to  repeat  the  oi^eration  six  months  later,  and 
so  on  from  year  to  year  till  the  columns  were  bowetl  percep- 
tibly out  of  line. 

In  all  towers,  in  each  plane  of  the  main  panel  points  of  the 
bracing  there  should  be  a  horizontal  system  of  diagonal  ad- 
justable rods  to  bring  the  columns  and  tower  to  place  and  line 
and  to  retain  them  there.  The  use  of  these  systems  of  hori- 
zontal bracing  is  a  sine  qua  iion  in  scientific  designing,  for 
their  omission  will  permit  the  faces  of  the  towei  to  get  out  of 
plane,  and  thus  the  metal  in  the  columns  will  become  over- 
strained. 

Whether  it  is  better  to  arrange  the  column  feet  of  towers  so 


m 


T)E   FONT  I  BUS. 


m  to  permit  of  tinintorrupted  expuiision  liud  contmction  or  to 
anchor  them  iluwii  fixedly  is  a  muuted  (lueHtion  amoug  en- 
gineers. Tlie  Hiitiiur  prefers  llie  former  method  for  the  reuson 
Ihut,  if  uil  tlte  feet  are  anchored  so  as  to  prevent  all  motion, 
either  tlie  pedestals  will  be  sprung  laterally  or  the  liorizontal 
struts  will  bullae  or  be  overstrained  when  the  temperature  is 
at  its  upper  extreme  range.  In  determining  the  metliod  of 
sliding,  one  foot  of  the  four  should  be  made  tixed  in  botli  di- 
rections, two  (should  be  tixe(]  in  one  direction  only,  and  the 
fourth  sliould  '     free  to  slide  in  both  rectangular  directions. 

Occasionally  it  is  advisabk-  to  use  hinged  ends  for  a  solitary 
Ix.-ut ;  but  the  author  generally  prefers  to  lix  the  feet  and  l>'t 
the  coliunn  spring  laterally  under  changes  of  temperature, 
'•iking  care  thtit  it  be  proi)()rtioned  properly  to  resist  tl:e 
stresses  due  t(»  such  springing  when  (he  s.ime  are  combined 
with  the  other  stresses  to  wliich  the  column  is  subjected. 
Fixed  ends  for  columns  of  solitary  bents  are  much  more  con- 
ducive  to  rigidity  of  structure  than  are  hinged  ends. 

The  question  of  sliding  ends  for  longitudinal  girders  will 
be  treated  in  the  next  chapter,  which  will  deal  with  clev;iled 
railroads,  the  expausi(m  i)ocket8  being  the  same  for  such 
structures  lis  for  railroad  trestles. 

The  best  sections  for  colunuis  are  either  two  channels  laced 
or  four  Z  bars  with  a  web  plate  or  lacing.  If  the  columns 
have  to  carry  transverse  loads,  they  should  have  solid  webs 
instead  of  lacing,  so  as  to  transmit  the  siiear  elfectively  from 
top  to  bottom.  For  light  work,  four  angles  in  the  form  of  an 
I  with  a  single  line  of  hieing  will  suffice. 

All  columns  when  spliced  should  have  their  splices  located 
about  two  feet  above  the  panel  points  of  the  column  bracing. 
Failure  to  so  locate  them  will  add  materially  to  the  cost  of 
erection.  All  such  splices  should  be  made  full,  more  espe- 
cially when  the  tension  on  the  column  runs  high. 

In  proportioning  anchorages,  the  pedestal  weight  should  i)o 
made  not  less  than  twice  the  greatest  net  uplift  from  tlie 
column,  due  account  being  taken  of  the  buoyant  effort  of  the 
water  la  case  of  a  possible  submergence  of  pedestal. 


to 

t'U- 

soil 
III. 

ital 

is 

of 

(li- 

tiiu 


CHAPTER  VIII. 


ELEVATED  RAILROADS. 


'^1  E  author  has  liitely  written  for  the  AmeHcun  Society  of 
Civil  Kugineers  a  Iciigtliy  pnper  on  this  8ubj»^i:l.  It  has  been 
very  tlioroughly  distMissed  by  the  engineering  profession,  and 
tlic  discussions  have  been  answered  in  an  cxliaustivu  resume 
by  tlie  author  and  his  assistant  engineer,  Ira  G.  Iledriclf, 
Assoc.  M.  Am.  Sor.  C.  E.  The  original  paper,  the  discus- 
sions, and  the  resume  hav<;  been  published  in  the  Transactions 
of  the  Society  for  1897,  Vol.  XXXVII ;  P'ld  any  one  who  de- 
sires to  make  a  special  study  of  the  subject  of  elevated  rail- 
roads will  do  well  to  read  all  that  has  been  published  thereon 
in  tlie  said  Transtictions. 

There  will,  however,  be  given  in  tliis  chapter  a  compen- 
dium of  tlie  contents  of  tlie  paper  for  the  use  of  those  who 
have  no  time  or  inclination  to  wade  through  the  two  hundred 
pages  that  it  occupies. 


Live  Loads. 

The  proper  live  load  to  assume  in  designing  an  elevated 
railroad  is  the  greatest  that  can  ever  come  upon  it,  and  is  de- 
termined by  ascertaining  the  weights  of  engines  and  empty 
cars  that  are  adopted  at  the  outset,  then  computing  how  many 
passengers  can  be  crowded  into  the  latter  and  assuming  that 
the  average  weight  per  passenger  is  one  hundred  and  forty 
pounds.  The  live  loads  for  elevated  railroads,  unlike  those 
for  surface  railroads,  do  not  increase  from  time  to  time,  but 
remain  constant.  In  fact  the  late  tendency  to  operate  the 
roads  by  electricity  rather  decretises  tnem,  for  the  weight  on 

91 


92 


I)E    PONTIBUS. 


lUe  axles  of  a  motor  car  produces  smaller  bending  moments 
than  that  ou  the  axles  of  a  locomotive. 

After  the  distribiUioii  of  the  live  load  on  the  various  axles 
of  the  entire  train  has  been  determintd,  it  is  well  to  prepare  a 
diagram  of  equivalent  uniform  loads  and  one  of  total  end 
shears  similar  to  those  for  the  Compromise  Standard  System 
of  Live  Loads  for  Railway  Bridges  given  in  Chapter  XIX, 
in  order  to  facilitate  the  computing  of  stresses  and  bending 
moments. 

Floor. 

The  style  of  floor  in  general  use  on  elevated  railroads  con 
siHts  of  timber  ties  with  four  lines  of  timber  guard-rails, 
dosed  floors  of  buckled  plate  carrying  timber  ties  in  baliiist 
being  employed  at  crossings  of  important  streets  and  boule- 
vards, so  as  to  prevent  dirt  and  moisture  from  falling  upon 
l)eople  passing  beneath.  Such  a  closed  floor  has  been  advo- 
cated  for  the  entire  line,  and  certainly  it  would  be  an  improve- 
ment upon  the  oi)en  floor  ;  l)ut  the  increased  expense  involved 
is  likely  to  interfere  seriously  with  its  adoption  for  future 
elevated  railroads.  The  ballast  over  the  buckled  i)late  in 
tight  floors  is  necessary  to  prevent  noise  from  passing  trains, 
whicrli,  unless  some  effective  sound-deadener  be  adopted, 
would  be  simply  deafening.  There  is  one  important  inci- 
dental advantage  in  employing  a  closed  floor,  viz. ,  that  tie 
elevation  of  the  grade  is  tliereby  re<luced  about  three  ftef. 
Of  course  nearly  as  great  a  reduction  of  elevation  can  be  ob- 
tained with  the  open  floor  by  resting  the  timber  ties  on  tlie 
inner  bottom  flanges  of  the  longitudinal  girders  ;  but  this  stylo 
of  structure  is  ol)jectional)]e  for  several  important  reasons, 
prominent  among  which  are  the  necessarily  large  sections  ul' 
the  ties  and  the  difficulty  in  replacing  them. 


Economic  Span  Lengths. 

With  the  ordinary  live  loads,  for  structures  located  on 
private  properly  the  economic  span  length  is  about  forty  feet, 


ELKVATKD    RAILROADS. 


93 


wbile  for  structures  lociited  in  the  sin.ei  it  varies  from  forty- 
seven  to  tifiy-tbree  feet  acconliiig  to  tlio  truiisverse  distance 
between  verticiil  axes  of  columns,  the  grealei  the  distance 
tlie  greater  the  economic  span  length.  With  heavier  live 
loads  the  economic  span  lengths  would  be  shorter. 

Foi:h-column  versus  Two-column  Structures. 

In  four-track  struclures  located  on  private  property  there 
is  but  lillle,  if  any,  difference  in  the  cost  whether  four  col- 
umns or  two  columns  per  bent  l)e  employed  ,  but  preference 
is  given  to  the  former  on  account  of  rigidity. 

-^      Braced  Towers  versus  Solitary  Columns. 

In  struclures  on  private  property  tliere  is  (luitc  a  gain  in 
both  rigidity  and  economy  by  adopting  braced  towers  spaced 
about  one  hundred  and  fifty  feet  centres. 

Rails. 

The  author  |)refers  to  adopt  for  elevated  railroads  steel 
rails  live  inches  high  weighing  not  less  than  eighty  pounds  to 
the  yard,  so  as  to  provide  for  tiie  excessive  wear  caused  by 
the  constantly  passing  trains. 

Treated  vermm  Untreated  'J'immek. 

Extended  investigations  have  proved  to  the  author's  snlis- 
fjiction  tiiat  it  pays  well  to  preserve  the  track  timber,  and 
that,  up  to  the  present  time,  by  far  the  best  preservative  pro- 
cess is  vulcanizing. 


Pedestal-caps. 

The  most  satisfactory  and  economicnl  pedestal-caps  are  of 
rpucrete  covered  with  at  leiust  six  inches  of  Iji'st-class  grajjit- 


94 


DE    PONTIBUS. 


Old.  These  have  all  the  advaotajjes  of  ciit-stone  blocks,  and 
are  generally  cheaper.  Tlie  latter,  howiver,  can  be  used  if 
there  be  anything  to  be  gained  thereby,  provided  lijat  the 
quality  of  the  stone  be  flrst-class  in  every  particular. 

AiS'CnORAGES, 

The  author  prefers  to  anchor  columns  to  pedestals  l)y 
means  of  anchor-bolls,  either  two  or  four  per  colunui,  ac- 
cording to  whether  there  is  bending  on  the  latter  in  one  or 
in  two  directions,  extending  well  down  into  the  concrete  and 
held  therein  by  a  cast-iron  spider,  and  extending  well  up 
outside  of  the  column,  lo  which  Ihey  connect  ijy  means  of 
long  enclosing  plates  and  heavy  washers.  TIk;  l)oxed  spaces 
at  the  column  feet  should  always  be  filled  with  concrete  to 
prevent  the  collection  of  dirt  and  moisture. 

Plate  Gikders  reraus  Open- webbed,  Riveted  Giudeus. 

As  far  as  economy  goes,  there  is  no  material  difference 
between  plate-girder  work  and  open-webbed,  riveted  work  ; 
but  the  former  is  more  satisfactory  in  most  particulars,  the 
only  real  advantage  of  the  latter  being  that  it  is  more  sightly. 
On  this  account  it  is  preferable  for  structures  occupying  the 
streets,  while  plate-girder  work  is  more  advantageous  for 
structures  located  on  private  property. 


Crimping  of  Web-stiffenino  Angles. 

Investigjitiou  has  shown  that  it  is  economical  to  crimp 
intermediate  stiffening  angles  and  to  use  tillers  beneath  all 
end  stiffeners. 

Sections  for  Columns. 

The  best  section  for  columns  located  in  the  street  is  com. 
posed  of  two  channels  with  their  llanges  turned  inward  aiul 
an  I  beam  riveted  between  the  channels,  ibe  ilanges  of  the 


ELEVATED    RAILROADS. 


95 


latter  being  held  in  place  by  interior  stay-plutes  spaced  about 
three  feet  centres.  Tlie  nmin  object  in  turning  the  flanges 
inward  is  to  enable  the  column  belter  to  resist  impact  from 
heavily  loaded  vehicles. 

The  most  satisfactory  section  for  columns  located  on  private 
property  consists  of  four  Z  bars  and  a  web  plate. 

Expansion  Joint. 

The  author's  ideal  expansion  pocket  is  described  very  fully 
by  both  text  and  drawings  in  his  paper  on  Elevated  Railroads, 
to  which  the  reader  is  referred. 


-A  Proper  Distance  between  Expansion  Points. 

With  columns  fixed  at  both  top  and  bottom,  as  the  author 
recommends,  the  proper  distance  between  expansion  points  is 
about  one  hundred  and  fifty  feet. 

SUPEKELEVATION  ON   CuRVES. 

Superelevation  of  the  outer  rail  can  be  obtained  by  varying 
the  heights  of  the  stringers,  by  putting  a  wooden  shim  on  the 
outer  stringer,  by  using  bevelled  ties,  or  by  spiking  a  shim  to 
each  tie.  The  last  two  methods  are  generally  preferable,  but 
tlie  second  one  can  occasionally  be  used  to  advantage,  while 
the  first  one  would  give  unnecessary  trouble  in  the  shops.  It 
will  generally  suffice  to  employ  only  three  bevels  for  ties,  viz., 
one,  two,  and  three  inches  in  five  feet.  Such  bevels  will  not, 
it  is  true,  afford  the  theoretical  superelevation  required  for  the 
maximum  speed  on  sharp  curves  ;  but  it  must  be  remembered 
that  it  is  (littlcult  to  maintain  high  speed  on  sharp  curves, 
hence  the  compromise  between  theory  and  practice. 


Faults  in  Existing  Elevated  Railroads. 

In  concluding  his  before-mentioned  paper,  the  author  made 
a  list  of  the  jiriucipal  faulty  details  in  existing  elevated  rail- 


d6 


DE   PONTIBUS. 


roads,  thei\.by  provoking  much  animated  discussion  ;  and,  as 
the  subject  is  one  of  great  importance  to  tlie  designer  of  future 
similar  structures,  that  portion  of  tlie  paper  which  inciudes 
this  list  will  be  reproduced  here  verbatim. 


1.    1N8UFFICIKNCY   OF    KIVKTS  FOH  CONNECTING  DIAGONALS 
TO   cnOKUH  OF  OPEN-WEUKKU,    KIVICTEI)   OIUDERS. 

This  flefecl  is  more  notietable  in  old  structures  than  in  later 
ones,  especially  as  the  tendency  nowadays  is  very  properl}' 
to  substitute  i>l)ile-girder  for  open-wt-bbed  construction.  In 
many  of  tlie  older  elevated  roads  there  is  no  connecting  pliite 
between  the  diagonal  and  the  ciiord,  l)ut  one  leg  of  each 
of  tiie  angles  in  the  diagonal  is  riveted  directly  to  the  vertical 
legs  of  the  chord  angles.  This  detail  involves  the  use  of 
either  two  or  four  rivels  to  the  connection,  which  is  evidently 
very  bad  designing,  as  there  should  be  more  rivets  used,  even 
if  the  diagonal  stresses  do  not  call  for  more  on  purely  theo- 
retical consideration.s.  Wlieie  the  theoretical  number  of  rivels 
is  very  small,  additional  rivets  should  be  used  for  two  reasons, 
viz. :  first,  one  or  more  of  the  rivets  are  liable  to  be  logse,  and, 
second,  there  is  nearly  always  a  torsional  moment  oa  each 
group  of  rivets,  owing  to  eccentric  coniiectiou. 


n.   FAILURE    TO  INTERSECT   DIAGONALS    AND   CHORDS   OP 
OPEN-WEBHED   GIRDERS  ON   GRAVITY   LINES. 

It  is  very  seldom  indeed  that  the  designer  even  attempts  to 
intersect  at  a  single  point  all  of  the  gravity  lines  of  members 
assembling  at  an  apex.  Tlie  failure  to  do  so  involves  large 
secondary  stresses,  especially  in  the  heavier  members.  By 
using  connecting  plates,  it  is  always  practicable  t(»  obtain  a 
proper  intersection  ;  and  it  is  always  better  to  do  this  than  to 
try  to  compensate  for  the  eccentricity  by  the  use  of  extra 
metal  for  the  main  members. 


III.    FAILURE  TO  CONNECT   WEB   ANGLES  TO  CHOI  DS   BY 

BOTH    LEGS. 

Some  standard  bridge  specitications  stipulate  tluit  in  case 
only  one  leg  of  an  angle  be  connected,  that  leg  only  shall  be 
counted  as  acting,  although  this  stipulation  is  generally 
ignored  by  the  designer  working  under  such  specifications. 


ELEVATKJJ    UAILKOAUS. 


It  is  seldom,  indeed,  tliat  bulli  Icj^s  are  cotiiiecled.  In  uiclur 
to  settle  llie  question  of  ihe  necessity  for  this  reqiiinjinent,  the 
author  has  liad  madi',  in  connection  with  his  North weslcrn 
Elevated  work,  a  series  of  tests  to  deslruclion  of  full-sized 
meuibers  of  opcn-webljed  girders,  attached  in  the  testing 
nmchiue  as  nearly  as  pradicable  in  the  same  way  as  they 
would  be  attatliecl  in  the  structure.  It  was  intended  to  settle 
by  these  tests  the  following  points  ;  rtrst,  effect  of  connccliug 
by  one  leg  only  ;  second,  effect  of  eccentric  connection  ;  and, 
third,  the  ultimate  strength  of  star  struts  with  tixed  ends,  each 
of  these  struts  being  formed  of  two  angles.  As  these  tests  are 
not  yet  finished,  their  results  cannot  be  given  here.  The 
principal  deduction  to  be  made  from  tlie  tests  thus  far  com- 
pleted is  tiiat  an  equal-legged  angle  riveted  by  one  leg  only 
will  develop  about  75;*  of  the  strength  of  the  entire  net 
section,  while  a  G'  X  3^"  angle  riveted  through  the  longer  leg 
wi]|  develop  about  90;*.  It  is  therefore  more  (conomical  lor 
short  diagonals  to  use  unequal-logged  angles  connected  by 
tlie  longer  leg  than  to  employ  supplementary  angles  to  try  to 
develop  liie  full  strength  of  the  piece.  In  fact,  the  ex|)eri- 
ments  made  up  to  date  indicate  that  these  supplementary 
angles  will  not  strengthen  the  diagonal  essentially  However, 
further  experiments  may  show  the  contrary. 

IV.  FAII-UKE  TO  I'llorOKTION  TOP  CHOllDS  OF  OPEN-WEUBED, 
LONGITUDINAL  OIHDEUS  TO  KE8IST  HFCNDING  FUOM  WUEEI. 
LOADS    IN     AUDITION     TO    THEIR     DIRECT     COMPRESSIVE 

8TKE88E8. 

This  neglect  is  common  enough  iu  the  older  structures,  and 
the  fault  is  a  serious  one,  although  the  stiffness  of  the  track 
rails  and  that  of  the  ties  tend  to  distribute  the  load  and  thus 
reduce  the  bending. 

V.    INSUFFICIENT   BRACING  ON  CURVES, 

Too  often  in  the  older  structures  the  curveil  portions  of  the 
line  are  no  better  braced  than  are  the  straight  jxjrtions.  A 
substantial  .system  of  lateral  bracing  on  curves  extending  over 
the  entire  width  of  the  structure  and  carrii'd  well  into  the  tops 
<tf  the  columns  adds  greatly  to  the  rigidity  of  t!;c  structure, 
and,  consequently,  to  the  life  of  the  metaUv.-ork. 


VI.     INSUFFICIENT     BRACING     lUiTWEEN 
LONGITUDINAL    (ilRDKRB. 


ADJACENT 


The  function  of  the  bracing  between  h)ngitudina]  girders  !,<> 
au  iiuportaut  one,  for  it  is  the  lirst  part  of  the  metal-wurk  l(; 


98 


DE   PONT  I  BUS. 


resist  the  swiiy  of  trtiins.  Not  only  should  the  top  flanges  of 
adjaceut  girders  be  connected  by  rigid  lateral  bracing,  bul  the 
bottom  flanges  should  be  stayed  by  occasional  cross-l)rHcing 
frames,  one  of  the  latter  being  invariably  used  at  each  ex- 
pansion end  of  each  track. 


VII.   PIN- CONNECTED,  P()NY-TRIJ88  PPAN8  AND  PLATE  GIRDERS 
WITH   UN9TIFFENKD  TOP  FLANGES. 

These  defective  constructions  are  noticeable  in  some  of  the 
older  lines,  but,  fortunately,  n(>t  often  in  the  newer. 

What  the  ultimate  resislaiuc  of  the  pony-truss  structure  is 
no  man  can  tell  without  ti-sling  it  to  destruction  ;  but,  in  the 
opinion  of  most  engineers,  it  is  nuicli  less  than  it  is  assumed  to 
be  by  those  designing  pony-truss  bridges. 


VIII.    KXCESS  OF   KXPANSION   .TOINTH. 

Too  many  expansion  joints  in  an  elevated  railroad  are  nearly 
as  bad  as  too  few.  In  the  former  cast;  the  melal  is  ovenstrained 
by  the  vibration  induced  by  the  lack  of  rigidil}',  while  in  tiie 
latter  case  it  is  overstrained  by  extreme  variations  of  tem- 
perature. There  are  elevated  roads  in  existence  with  expan- 
sion joints  at  every  other  bent,  and  there  is  at  least  one  with 
them  at  every  bent.  For  long  spans  there  slnndd  be  expan- 
sion provided  at  every  third  bent,  and  for  short  spans  al  every 
fourtli  bent. 


IX.  RESTING  LONGITUDINAL  OIUDKUS  ON  TOP  OF  CROSS- 
GIRDKRS  WITHOUT  RIVETING  THKM  EFFECTIVELY 
THERETO. 

This  is  l)y  no  means  an  uncommon  detail,  especially  in  the 
older  structures.  It  is  conducive  to  vibration,  and  its  only 
advantages  are  ease  of  erection  and  a  cheapening  of  the  work 
by  avoiding  tield-riveting. 


X.    CROSS-GIUDERS  SUBJECTED    TO    HORIZONTAL    BENDING 
THRUST   OF   TRAINS. 


BT 


The  resistance  that  can  be  offered  by  a  cross-girder  to  hori- 
zontal bending  is  very  small ;  nevertheless,  cio-ss-girders  are 


ELKVATED    RAILROADS. 


9D 


rarely  protected  from  the  bending  effects  of  thrusts  of  trains. 
Wlial  saves  these  cross-girders  from  faihire  is  the  fact  that 
coniiimity  of  tlie  Iraclc  tends  to  distribute  the  thrust  over  a 
II umber  of  bents.  Nevertheless,  it  is  not  legitimate  to  depend 
on  this  fact,  lor,  especially  on  sharp  curves,  the  tendency  is  to 
carry  the  thrust  into  the  ground  as  dinctly  as  possil)le.  in  the 
author's  opinion,  tiie  only  propir  way  to  provide  for  this  thrust 
is  to  assume  that  20%  of  the  greatest  live  load  between  Iwoad- 
jacHiit  expansion  points  will  act  as  a  horizontal  thrust  upon 
ihc  columns  betwein  these  Iwoexpansion  points  ;  and  all  parts 
of  tht!  metal-work  should  be  proportioned  to  resist  this  thrust 
properly. 

liy  running  a  strut  from  the  top  of  each  post  diagoiuilly  to 
th(!  longitudinal  girder  at  a  panel  point  of  its  sway-bracing, 
the  horizontal  thrust  is  carried  directly  to  the  post,  and  a  lior- 
izont^l  bending  moment  on  the  cross-girder  is  thus  prevented. 
Such  (construction  should  invariably  be  used  where  the  condi- 
L'ons  require  it. 


XI.    CUTTING  OFF  COLU.MNR    HKT-OW    THK    BOTTOM    OF    CKOS8- 
(JIUDKUS   AND   KESTINd   THK    LATTRK   THEHKON. 

This  style  of  construction,  which  until  lately  was  almost 
universal,  is  extremely  faulty  in  that  there  is  no  rigidity  iu 
the  connection,  and  the  culumu  is  thus  made  more  or  less  free- 
ended  al  tlie  top. 

It  has  been  said  that  no  harm  is  done  to  the  colunui  by 
making  it  free  ended,  as  it  can  then  spring  belter  when  the 
thrust  is  applied.  Unfortunately  this  reas')ning  is  fallacious, 
because  the  few  unlucky  rivets  which  connect  the  bottom  of 
the  crosH.girder  to  the  lop  of  the  column  lend  to  produce  a 
fixed  end,  and  arc,  in  consequence,  racked  excessively  by  the 
thrust  of  the  train.  In  all  cases  the  column  should  extend  to 
the  top  of  the  cross-girder,  and  should  be  riveted  to  it  iu  the 
most  effective  manner  practicable. 


XII.    PALTRY  BRACKETS  CONNECTINO   CROSS-GIRDERS  TO 

COLUMNS. 


Brackets  are  often  seen  compo.sed  of  a  couple  of  little  angles 
attached  at  their  ends  by  two  or  three  rivets.  Such  brackets 
are  merely  an  aggravation,  and  are  sure  to  work  loose  sooner 
or  later.  Althougli  it  is  impracticable  to  compute  the  stresses 
in  this  detail,  good  judgment  will  dictate  the  use  of  solid- 
webbed  brackets  riveted  ri^gidly  to  both  cross jjirder  and  CQJ- 


100 


DE   P0NTIBU8. 


umii  so  m  to  stiffen  the  latter  and  check  the  transverse  vibra- 
tion  from  pnssiug  trains. 


XIU.  PROPOKTIONING  COLUMNS  FOU  DIRECT  LIVE  AND 
DEAD  LOADH  AND  IGNORING  THE  EFFECTS  OF  BENDING 
CAUSED  BY  THRUST  OF  TRAINS  AND  LATEUAL  VIBRA- 
TION. 

The  practical  effects  <j{  this  fault  can  be  seen  to  best  advan- 
tage by  standing  on  one  of  the  high  platforms  of  one  of  the 
elevated  railroads  of  New  York  City.  The  vibration,  by  no 
means  small,  from  an  approaching  train  can  be  felt  when  it  is 
yet  at  a  great  distance.  Some  may  claim  that  this  /ibratioii  is 
not  injurious;  but  they  are  certainly  wronir,  for  what  docs  it 
matter,  so  far  as  the  stress  in  the  column  is  concerned,  whether 
the  deflection  be  caused  by  vibration  or  by  a  statically  applied 
transverse  load,  so  long  as  the  amount  of  the  deflection  is  the 
same  in  both  cases?  It  takes  metal,  and  considerable  of  it,  lo 
make  columns  strong  enough  to  resist  bending  properly  ;  and 
a  sufficient  amount  should  be  used  to  attaiu  this  end. 


XIV.    OMISSION  OF  DIAPHRAGM  WEBS   IN  COLUMNS   SUBJECTED 

TO  BENDING. 

If  the  diaphragm  "web  be  omitted  in  such  a  column,  reliance 
must  be  placed  on  the  lacing  to  carry  the  horizontal  thrust 
from  top  to  bottom.  But  even  if  the  lacing  flgure  Ptrong 
enough  to  carry  it,  which  is  unusual,  it  is  wrong  to  assume  it 
so,  for  the  reason  that  one  loose  rivet  connecting  the  lacing- 
bars  will  prevent  the  whole  system  from  acting,  as  will  also  a 
lacing-bar  that  is  bent  out  of  line.  Decidedly  every  column 
that  acts  as  a  beam  also  should  have  solid  webs  at  right  angles 
to  each  other. 


XV.   INEFFECTIVE  ANCHORAGES. 


On  account  of  both  rigidity  and  strength,  every  column 
ought  to  be  anchored  so  flrmly  to  the  pedestal  that  failure  by 
overturning  or  rupture  woidd  notocjurin  the  neighborhood 
of  the  foot,  if  the  bent  were  tested  to  destruction.  The  flimsi- 
ness  of  the  ordinary  column- footconuectiou  is  beyoud  descrip- 


ELEVATKD   RAILROADS, 


101 


XVI.   COLUMN   FEET   SUltUOUNDED  BY  AND  FILLED  WITH 
DIRT   AND  MOISTURE. 

The  condition  of  the  ivvcmge  cohimn-foot  ia  simply  deplor- 
able. This  is  caused  by  fuiling  to  mise  it  so  high  nhove  the 
street  as  to  pieveul  dirt  from  piling  Hrouiid  il,  and  by  omitting 
to  fill  its  boxed  spaces  with  concrete.  Wlicn  rusting  at  a 
column-foot  is  once  well  started,  it  is  almost  impossible  to 
stop  it  from  eating  up  the  metal  rapidly. 


XVII.    INSUFFICIENT   BASES   FOR   PEDESTALS. 

False  ideas  of  economy  on  the  part  of  projectors  and  indi- 
ference  on  the  part  of  some  unscrupulous  contractors  occasion- 
ally cause  the  use  of  pedestal  bases  altogether  too  small  for 
the  loads  that  come  upon  them,  especiall}'  where  the  bearing 
capacity  of  the  soil  is  low.  The  result  is  sun:  en  pedestals 
and  cracked  metal-work.  In  figuring  the  prtssure  on  the 
base  of  the  pedestals,  it  is  not  sufilcieut  to  recognize  only  the 
direct  live  and  dead  loads,  but  il  is  neces.sary  also  to  compute 
the  additional  unequal  intensities  of  loading  caused  by  both 
longitudinal  and  transverse  thrusts. 

Concerning  the  question  of  the  extent  to  which  the  faults 
just  outlined  exist  in  the  older  elevated  railroads  of  this 
coiintry,  the  author  would  refer  the  reader  to  the  resume  of 
discussions  on  his  paper,  and  to  the  report  of  Mr.  Hedrick 
which  it  contains. 

About  the  most  important  object  to  attain  in  constructing 
an  elevated  railroad  is  to  have  a  perfectly  smooth  and  durable 
track;  and  no  trouble  or  expense  should  be  spared  to  secure  it. 
For  this  reason  the  top  flanges  of  the  longitudinal  girders,  if 
the  limiting  heights  of  grade  and  clearance  line  permit,  should 
be  several  inches  higher  than  those  of  the  cross- girders,  the 
ties  should  all  be  planed  to  exact  dimensions  tie-plates  should 
be  used  over  all  ties,  anil  the  system  of  bolting  of  flooring  to 
structure  should  be  the  most  effective  p^^  *ible.  The  longi- 
tudiruil  girders  should  not  be  made  continuous,  or  even  semi- 
continuous  over  the  cross-girders,  but,  when  blocking  up  is 
necessjiry,  short  buckled  plates  should  be  placed  over  the 
latter  so  as  to  provide  a  continuous  surface  for  the  ties.  Hook 
bolts  with  cold- pressed  threads  should  be  used  for  attaching 

PROVsNO-M.    t-IDRARY 


102 


DE  POKTIBUS. 


tbe  timber  to  the  mettil-work  through  each  alternate  tic,  the 
other  ties  being  l)olted  to  the  inner  guard-rails. 

The  ties  should  be  spaced  witli  openings  not  greater  tl)an 
six  inches,  their  section  for  a  five-foot  stringer  being  0"  X  H" 
laid  on  flat ;  but  where  cross-overs  are  employed,  the  depth 
should  be  properly  increased  to  withstand  the  bending  mo- 
ment due  to  the  greatest  load  from  the  wheels. 

The  least  allowable  overhead  clearance  for  most  cities  is 
fourteen  feet ;  but  there  are  sometimes  special  crossings  re- 
quiring a  greater  height.  The  width  of  right  of  way  beyond 
the  centre  line  of  the  outer  track  should  not  be  less  than  seven 
feet.  The  proper  depths  of  longitudinal  girders  are  to  be 
determined  very  carefully.  For  the  sake  of  appearance  it  is 
generally  uot  well  to  use  more  than  one  depth,  but  such 
an  arrangement  cannot  always  obtain.  The  general  depth 
should,  if  possible,  be  the  economic  one  for  the  average  span 
length.  For  plate-girder  spans  it  is  about  one  twelfth  of  the 
length,  while  for  open  -  webbed,  riveted  spans  it  is  much 
greater — so  much  greater,  in  fact,  that  for  deck-spans  the 
economic  depth  cannot  be  adopted,  because  of  the  raising  of 
the  grade  which  would  be  caused  thereby. 

Before  the  designing  of  the  metal-work  for  an  elevated 
railroad  is  started  there  are  certain  important  matters  which 
should  be  fully  determined,  viz.,  the  dimensions  and  weights 
of  rolling  stock,  sizes  and  number  of  trains,  method  of  trac- 
tion and  the  proper  track  to  suit  same,  the  locations  of  all  sta- 
tions and  their  leading  dimensions,  the  storage  capacity  for 
the  terminals,  the  capacity  of  the  repair-shops,  and  the  method 
of  operating  the  road.  Unlesy  all  these  questions  be  settled 
conclusively  at  the  outset  and  before  the  designing  is  begun, 
trouble  is  sure  to  ensue  because  of  changes  that  will  have  to 
be  made  from  time  to  time  during  the  course  of  construction. 

In  designing  elevated  railroads  according  to  tlie  specifica- 
tions given  in  this  treatise,  it  must  not  be  forgotten  that  the 
entire  line  is  to  be  proportioned  by  the  specifications  for  rail- 
road bridges  (Chapter  XIV),  while  the  stations  are  to  be  pro- 
portioned by  the  specifications  for  highway  bridges  (Chapter 
XVI). 


CIIAPTEH  IX. 


MOVABLE  imiI)OP:s  IN  GENERAL, 


MovABi.K  briilges  nmy  be  divided  into  the  followiug  eight 
types : 

1.  Ordinfuj,  rolatiiig  draws. 

2.  Doublf,  rolaling,  cjintilever  draws. 

3.  Pull  buck  draws. 

4.  Couiiterweightcd,  bascule  bridges. 

5.  Rolling,  bascule  bridges. 

6.  Jack-knife  or  folding  bridges. 

7.  Lifl-bridges. 

8.  Floating  bridges. 

The  ordinary  rotating  draws  will  be  treated  at  length  in  the 
next  chapter. 

Very  few  double,  rotating,  cantilever  draws  have  yet  been 
built;  in  fact  the  author  knows  of  but  one,  viz.,  that  over 
the  canal  at  Cleveland,  Ohio.  A  number  of  years  ago  the 
author  had  occasion  to  15gure  on  a  large  structure  of  this  kind, 
btit  it  was  never  built. 

The  principal  advantages  of  this  type  of  structure  are  a 
wide  waterway  and  the  retreating  of  the  span  without  serious 
injury  when  struck  by  a  vessel  before  it  is  fully  opened  ; 
while  its  disadvantages  are  excessive  first  cost  and  the  almost 
double  cost  of  operating  two  independent  spans ;  although, 
when  electricity  is  iised  as  a  motive  power,  both  spans  can  be 
operated  by  one  man  by  means  of  a  submerged  cable. 

This  class  of  bridge  consists  of  t  vo  draw-spans,  differing 
but  little  from  the  ordinary  rotating  draw,  each  resting  upon 
a  pivot-pier  and  meeting  at  mid-channel,  where  they  are 

108 


104 


I)R   P0NTIBU8. 


locked  lopt'iher  so  as  lo  iniike  tliu  adjuiiiiiig  ends  deflect 
equally  iiud  simulliiiieoiisl}'.  The  oilier  end  of  each  draw  is 
locked  to  the  masoniy  of  the  oiiler  rest-pier,  which  acts  as  an 
anchorage.  It  is  not  necessary  lo  make  the  siiore  anus  of  the 
same  length  as  Ihe  eliaiinel  arms  ,  but  if  there  be  a  difference, 
there  most  be  compensating  weight  so  as  to  balance  each 
s;)Uii  over  the  centre  of  the  pivot-pier,  and  there  must  be  a 
vertical,  close  surface  provided  at  the  end  of  each  short  arm 
so  as  to  equalize  as  well  as  possible  the  niometits  of  the  wind- 
pr(!ssure  on  the  two  arms.  This  class  of  bridge  is  probably 
not  very  rigid,  but  it  can  be  made  quite  satisfactory  and 
effective. 

The  pull-back  draw  is  also  a  very  unusual  type,  and  will 
always  be  so,  for  the  reason  thai  tlie  first  cost  is  great  and  its 
operation  is  expensive.  Tliis  ty|)e  may  be  divided  into  two 
chisses  lirst,  structures  with  one  «pan  over  the  entire  opening, 
and,  second,  structures  with  two  spans  over  the  entire 
opening,  meeting  at  midchanncl,  as  in  the  case  of  the  double, 
rotating,  cantilever  draw.  Tiie  first  class  requires  a  truss- 
bridge  nearly,  if  not  quite,  twice  as  long  as  the  width  of  chan- 
nel between  pier  centres,  the  bottom  chords  thereof  running 
on  two  groups  of  rollers  that  travel  just  half  as  fast  as  the 
bridge  wlien  the  span  is  moved  longitudinally.  Although  the 
shore  arm  may  be  made  shorter  than  the  channel  arm,  still  its 
weight  musl  be  such  that  its  moment  will  be  soniewhat  greater 
than  the  lipping  moment  jf  the  weight  of  the  channel  arm  ju^t 
as  it  leaves  tlie  farther  pier.  A  disappearing  platform  will  be 
required  so  as  to  leave  si)ace  on  the  approach  for  the  shore  arm 
to  move  back,  or  else  the  whole  bridge  will  have  to  be  rotated 
slightly  about  a  horizontal  axis  so  that  it  can  roll  up  onto  the 
approach.  Either  rsethod  is  very  clumsy,  and  the  operation 
of  the  bridge  con-sequently  must  be  slow. 

The  double  pull-back  draw  is  similar  to  the  single  pull-back 
draw  just  described,  except  that  the  far  end  of  each  span  has 
t.)  be  anchored  down  to  a  mass  of  masonry  when  the  bridge  is 
closed  and  ready  fci  traffic,  and  the  ends  meeting  at  mid-chan- 
nel must  be  locked  together  as  in  the  case  of  the  double, 
rotating,  cantilever  draw. 


MOVAHLK    BklDGF'S    IN"    OENEKAL. 


105 


Tlie  author  had  occasion  several  years  .'.go  to  design  a 
double,  pull-back  drawbridge  ;  and  aUI)0ugh  ho  certainly 
evolved  a  structure  that  would  work,  lie  was  far  from  siitisfled 
with  the  design,  so  recommended  anotlier  type  of  bridge  for 
the  crossing. 

There  Is  described  in  the  Engineering  Record  of  July  !J1, 
1897,  a  double  pull-biick  draw,  which  lias  just  been  completed 
over  the  River  Dee  at  Queensfeiry.  Scotland.  It  provides  a 
clear  opening  of  one  hundred  and  iwenty  feet,  and  cost  about 
$70,000. 

Bascule  bridges  are  those  In  which  a  shallow  deck  is  raised 
from  a  hori/.oiitnl  position  to  a  vertical  or  inclined  one  so  as  to 
let  vessels  pass.  They  miiy  have  either  one  or  two  leaves 
wiiose  weight  may  be  counterbalanced  in  various  ways.  When 
two  leaves  are  used,  they  may  be  made  to  me H  at  mid-chan- 
nel and  form  an  arch,  may  rest  on  a  cential  pier,  may  hang 
from  a  tower  or  from  an  overhead  span,  or  may  have  hanging, 
liinged  bents  to  rest  on  a  submerged  pier  at  the  elevation  of 
the  bed  of  the  channel. 

For  spans  requiring  leaves  not  longer  than  seventy-five  feet 
the  bascule  type  of  bridge  is  very  satisfactory;  but  bej'ond 
that  limit  the  first  cost  of  the  structure  begins  to  get  too  high 
as  comi)ared  with  anotlier  type  of  equally  satisfactory  struc- 
ture, vi/,.,  the  lift-bridge.  Some  basctile  bridges,  notably  the 
Tower  Bridge  of  London,  England,  have  an  overhead  span  to 
be  used  in  connection  with  elevators  by  pedestrians  when  the' 
lower  deck  is  opened  for  the  passage  of  vessels.  The  leaves 
of  the  Tower  Bridge,  which  are  each  113  feet  long,  do  not 
raise  quite  to  a  vertical  position,  requiring  one  and  a  half 
minutes  to  open  and  as  much  more  to  close  under  favorable 
conditions  of  wind  and  weather,  and  sometimes  twice  as  long 
when  the  conditions  are  unfavorable.  The  author  has  been 
told  by  an  English  gentleman  resident  in  America,  who  made 
latdy  an  investigation  concerning  the  Tower  Bridge,  that  the 
London  people  complain  bitterly  about  the  long  time  it  takes 
to  operate  the  structure.  A  lift-bridge  similar  to  the  Halsted 
Street  Lift-bridge  of  Chicago,  111.,  could  have  been  built 
instead,  which  would  raise  to  full  height  in  from  thirty  to 


lori 


in:  I'ONTiBUS. 


t'orly-five  seconds  and  lower  aguiu  iu  the  same  lime.  A  full 
description  of  the  Tower  Bridge  is  given  iu  the  Proceedings  of 
the  Institution  of  Civil  Engineers,  Vol.  CXXVII. 

A  good  example  of  the  rolling  ba-seule  bridge  is  the  Van 
Buren  Street  Bridge  at  Clncago,  111.,  which  structure  is 
described  in  Engineering  Nt'ws  of  Feb  21,  1H95.  It  consists 
of  two  leaves,  each  about  seventy  feet  long,  ending  in  a 
cylindrical  surface  tliat  rolls  on  a  plane  i)rovi(led  with  teeth 
winch  gear  into  the  roller  to  prevent  slipping.  When  the 
bridge  is  closed  the  short  end  of  each  arm  is  anchored  down  to 
the  masonry  so  as  to  permit  of  its  acting  as  a  cantilever. 

Close  alongside  of  this  structure  is  the  Metropolitan  Elevated 
Railroad  Company's  four-track  bridge,  whicli  is  also  of  ll)e 
rolling  bascule  type.  This  is  divided  into  two  .similar  double- 
track  bridges  placeil  close  together  and  operated  separately, 
so  that,  in  case  of  acciilent  to  one  bridge,  t!ie  lailroad  tnithc 
may  be  diverted  to  the  other,  while  the  injured  span  is  raised 
out  of  the  way  of  the  river  Iralllc. 

It  is  seldom  advisable  to  use  a  centre  pier  to  rest  the  leaves 
of  tlie  bascule  upon,  on  account  of  the  obstruction  which  it 
would  oiler  to  navigation, 

A  submerged  pier  to  receive  the  ends  of  the  posts  of  hang- 
ing hinged  bents  has  never  been  u.sed,  nor  is  it  at  all  likely 
that  it  ever  will  Ix;,  owing  to  the  diniculties  that  wimkl 
be  (Micountercd  in  operation,  such  as  those  from  ice,  drift- 
ing sand,  changing  currents,  etc.,  all  of  which  would  tend 
to  prevent  the  column-feet  from  taking  proper  bearing  on  the 
pier. 

Suspending  the  ends  of  the  leaves  from  an  overhead  span,  or 
tying  them  back  to  the  tops  of  towers,  is  a  perfectly  feasible 
method,  but  is  expensive  and  without  adv.intage. 

There  is  a  bascule  itridge  in  Ciiicago  which  is  counter- 
weighted  by  four  nnu's-ses  of  cast  iron  in  carriages  that  run  upon 
curved  surfaces  on  the  iipproaches,  tlie  curves  Iwing  so  figured 
that  the  varying  load  at  the  channel  end  of  the  leaf  is  at  all 
times  balanced  by  the  varying  tension  on  the  cables  which  hold 
ilie  counterweights. 

There  is  a  similar  structure  on  Michigan  Avenue  in  Bullalo, 


MOVAHLK    HIIIDCJKS    I.V    OKNfKUAL. 


lot 


full 
s  of 


N.  Y.,  which  is  described  in  the  Engineering  Record  oi  Aug. 
31,  1897. 

Several  years  iigo  the  author  figured  on  a  bridge  of  tliistype, 
but  abandoned  Uie  design  because  he  deemed  it  inferior  to 
several  others  which  lie  prepared  for  the  same  crossing. 

An  excellent  type  of  bascule  bridge  is  that  of  the  Sixtecntli 
Street  Bridge  over  the  Menominee  Canal  at  Milwauke*^,  Wis. 
It  is  described  in  Engineering  Newa  of  ]\[arch  7,  1895.  The 
peculiar  feature  of  this  design  is  that,  during  motion,  tlie 
centre  of  gravity  of  the  mass  travels  in  a  horizontal  plane, 
thus  reducing  to  zero  the  lifting  effort  for  tlie  machinery. 

A  temporary  bascule  of  peculiar  detail  was  used  for  several 
years  at  the  crossing  of  the  Harlem  River  on  the  Is'ew  York 
Central  and  Hudson  Uiver  Railroad.  It  is  described  in  the 
ll'iilroad  (utzette  of  June  10,  1892.  Tlie  characteristic  feature 
of  thii*  structure,  which  by  the  way  is  a  rather  clumsy  con- 
trivance, is  the  picking  up  and  dropping  of  small  counter- 
wcigiits  while  lowering  or  raising  the  span. 

Tliere  is  a  serious  objection  to  all  large  bascule  bridges,  viz., 
the"  great  surface  opposed  to  the  wind  by  the  leaf  or  leaves 
when  the  bridge  is  being  opened  or  closed.  To  overcome  this 
pressu  •('  powerful  machinery  has  to  be  used  ;  and  it  is  by  no 
means  iMiprobal)le  that  even  sucJi  machinery  will  be  stalled 
when  a  high  wind  })revails. 

The  jack-knife  or  folding  bridge  is  a  type  of  structiire  wliicl» 
is  not  at  all  likely  to  bcconie  conimoii.  There  have  been  only 
two  or  three  of  them  built  thus  far,  and  they  have  been  often 
out  of  order  ;  moreover,  considering  the  size  and  weight  of 
bridge,  the  nuidiinery  used  is  powerful  and  expensive. 
The  load  on  the  machinery  while  eitiier  opening  or  closing  the 
bridge  is  far  from  uniform,  and  tin;  structure  at  times  almost 
seems  to  groan  troui  the  liartl  laDor.  The  characteristic  feature 
of  the  jack-knife  bridge  is  the  folding  of  the  two  bascule 
leaves  at  mid-length  of  same  when  the  bridge  is  opened.  The 
loose-jointedness  involved  V)y  this  detail  is  by  no  means  con- 
ducive to  rigidity  ;  nevertheless  these  structures  are  stiller 
than  one  would  suppose  from  iiti  examination  of  the  drawings. 
Tlie   Canal  Street  IJridge,  Chicago,   is  of   this  type;  and  its 


108 


t)K    I'ONTIBUS. 


tk'sign  is  Illustrated  in  Engineering  News  of  December  14, 
1893. 

Lift-bridges  on  a  small  scale  have  been  used  for  many  years 
for  crossings  of  canals,  lifting  only  high  enough  to  let  the 
canal-boats  pass  benoalh.  They  have  proved  to  be  quite 
satisfactory  ami  fairly  economical  in  both  first  cost  and 
operation,  the  method  of  the  latter  being  usually  man-power. 

No  large  structure  of  this  type  was  ever  built  until  1893, 
■when  the  author  designed  for  the  city  of  Chicago  the  South 
Halsted  Street  Lift-bridge.  This  structure  lu\s  been  described 
in  the  principal  engineering  papers  of  America  and  Europe, 
and  the  author's  description,  written  for  the  American  Society 
of  Civil  Engineers,  may  be  found  in  the  Transnctiona  of  that 
Society  for  Januar}'  1895,  from  whicli  the  following  descrip- 
tion is  taken  : 

The  bridge  (  onsists  of  a  single,  Pratt  truss,  through-span  of 
130  ft.,  in  seven  equal  panels,  and  having  a  truss  depth  of 
23  ft.  between  centres  of  chord  pins,  ,so  supported  and  con- 
structed as  to  permit  of  being  lifted  vertically  to  a  height  of 
155  ft.  clear  above  mean  low  water.  At  its  lowest  jiosition 
the  clearance  is  about  15  ft.,  wiiich  is  sufficient  for  tlie  pas- 
sage' of  tugs  when  tlieir  smokestacks  are  lowered.  Tin;  span 
dilfers  from  ordinary  bridges  only  in  having  provisions  for 
attaeliing  the  sustaining  and  hoisting  cables,  guide-rollers, 
etc  ,  and  in  the  inclination  of  the  end  posts,  which  are 
battered  slightl}',  so  as  to  bring  tlicir  upper  ends  at  tlie  pro|)er 
distance  from  the  tower  cohunns,  and  their  lower  ends  in  the 
required  position  on  the  piers. 

At  eaclj  side  of  the  river  is  a  strong,  thorouglily  braced, 
steel  tower,  about  2!7  ft.  high  from  the  water  to  the  top  of 
tlie  housing,  exclusive!  of  tlu;  Hag-poles,  cariying  at  its  top 
four  btiilt-up  sl?el  and  cast-iron  sheaves,  12  ft.  in  diameter, 
which  turn  on  12-in.  a.vles.  Over  these  sheaves  pass  the  l^-ir. 
steel-wire  ropes  (32  in  all)  which  sustain  the  span.  Tlicse 
ropes  are  double,  i.e.,  two  of  them  arc,  brought  together  wliere 
tlie  span  is  suspended,  and  the  ends  are  fastened  by  clamps, 
while,  where  they  attach  to  tlie  coiinterwcighls,  they  form  a 
loop,  which  passes  around  a  15-in.  wheel  or  pulley  that  nets  as 
an  equalizer  in  case  the  two  adjacent  ropes  tend  to  stretch 
unequally. 

The  couuterweiglits,  which  are  intended  to  just  balance  the 
weight  of  the  span,  consist  of  a  number  of  horizontal  cast-iron 
blocks  about   10  x  12  in.  in   section,   and   8  ft.  7  in.  long, 


MOVAHLE    BRIDGKS    IX    (}KNEIIAL. 


100 


strung  on  adjustable  wrouglit-iroii  rods  tliat  are  attached  to 
the  ends  of  rockers,  at  tlie  middle  of  each  of  which  is  inserted 
the  15-in.  equalizing  vvlieel  or  pulley  previously  mentioned. 

The  counterweights  run  up  and  down  in  guide-frames  built 
of  3-'n.  angles. 

Tiie  weigiit  of  the  cables  is  counterbnlaneed  by  that  of 
wrought-iron  chains,  one  end  of  each  chain  being  attached  to  the 
span  and  the  oilier  end  to  the  counterweights,  so  that,  what- 
ever may  be  the  elevation  of  the  span,  there  will  always  be 
llie  same  combined  w(  iglit  of  stistaining  cables  and  chiun  on 
one  side  of  each  main  sheave  as  there  is  upon  its  other  side. 

Between  the  tops  of  tlie  oppo.-ite  towers  pass  two  shallow 
girders  thorouglily  sway-braced  to  e'M'h  other,  and  riveted 
rigidly  to  said  towers.  The  main  fiuiction  of  these  girders 
is  to  iiold  tlie  t()i)s  of  the  towers  in  correct  iiosition;  but  in- 
cidentally they  serve  to  support  the  idlers  of  the  operating 
ropes  and  to  aiford  a  footwalk  from  tower  to  tower  for  the  use 
of  the  bridge-tender.  Adjustable  pedestals  under  the  rear  legs 
of  each  lower  i)rovide  for  unequiil  settlement  of  the  i)iers  which 
support  the  tower  columns.  Each  of  these  pedestals  has  an 
octagonal  forged  steel  shaft,  expanding  into  a  si)here  at  one 
end,  and  into  a  cylinder  with  screw-threads  at  the  other.  The 
ball  end  works  in  a  sjjherical  socket  on  a  pedestal,  and  the 
screw  end  works  in  a  female  screw  in  a  casling  which  is  very 
firmly  attached  to  the  bottom  of  the  tower-leg.  By  turning 
the  octagonal  shaft,  it  is  evident  that  the  rear  column  will  be 
lengthened  or  shortened.  Tlie  turning  is  accomplished  by 
means  of  a  special  bar  of  great  si  length,  which  tits  closely  to 
the  octagon  at  one  end,  and  to  tin;  (ith..r  end  of  which  can  be 
connected  a  block  and  tackle  if  necessary.  These  screw  ad- 
justments were  useful  in  erecting  the  structure,  but  it  is  quite 
Ukely  that  they  will  never  again  he  needed.  But  in  case  there 
is  ever  any  tower  adjustmeiii  retiuired,  it  will  be  found  that 
the  extra  money  spent  on  tliem  will  have  l)een  well  expended. 

Each  tower  consists  of  two  vertical  legs,  against  which  the 
roller-guides  on  the  trus.ses  bear,  and  two  inclined  rear  legs. 
Tiicse  legs  are  thoroughly  braced  together  on  all  four  faces  of 
the  tower;  and  at  each  tier  tliereof  there  is  a  system  of  hori- 
zontal sway-bracing,  which  will  prevent  most  etleclively  every 
tendency  to  distort  '.lie  lower  by  torsion. 

At  the  tops  of  the  towers  there  are  tour  hydraulic  buffers 
that  are  capable  of  bringing  the  span  to  rest,  witliout  jar,  from 
its  greatest  velocity,  which  was  assumed  to  be  4  ft.  per  second; 
and  there  are  four  more  of  these  bulTcrs  attached  beneath  the 
span,  one  at  each  corner,  to  serve  the  same  |>iirpose. 

The  span,  with  all  that  it  carries,  weiglis  about  290  tons,  and 
the  counterweights  weigh,  as  nearly  as  may  be,  the  same.  As 
the  cables  ami   their  counterbalancing  chains  weigh  fullj'  20 


110 


DE    PONTIBUS. 


tons,  Ihc  total  weight  of  the  moving  mass  is  almost  exactly  600 
tons. 

Should  tiie  span  and  the  counterweights  hecome  out  of  bal- 
ance on  account  of  a  greater  or  less  amount  of  moisture,  snow, 
dirt,  etc.,  in  and  on  the  jtavement  andsitlewallvS,  it  can  be  ad- 
justed by  letting  water  into  and  out  of  hnllast-tanks  located 
lieneiith  the  floor,  and,  should  tlds  ndjustinent  be  in.'iurticient, 
provision  is  made  i'or  adding  small  weights  to  the  counter- 
weiglits,  or  for  placing  such  weigiits  on  the  span. 

As  the  counterweights  tints  l)alancc  the  weight  of  the  span, 
all  the  work  whicli  the  machinery  has  to  do  is  to  overcome  tlie 
friction,  bend  the  wire  ropes,  and  raise  or  lower  any  small  un- 
balanced load  that  there  may  be.  It  lias  been  designed,  liow- 
ever,  to  lift  a  considerable  load  of  passengers  in  case  of  neces- 
sity, although  tlie  structure  is  not  intendcnl  for  this  purpose, 
and  should  never  be  so  used  to  any  great  e.tlent. 

The  span  is  steadied  while  in  motion  by  rollers  at  tlie  lops 
ami  bottoms  of  the  trus.sos.  There  are  bolli  Iransver-se  and 
longitudinal  rollers,  the  former  not  touching  the  columns,  un- 
less there  is  sufficient  wiiul-pressure  to  bring  them  to  a  be.nr- 
ing.  The  longitudinal  rollers,  though,  areatla(;h(.'d  to  spri;igs, 
winch  press  them  against  the  columns  at  all  times,  and  take 
up  the  expansion  and  contraction  of  the  trusses.  With  the 
rollers  removed,  the  briilge  swings  free  of  the  columns  ;  and, 
since  the  attachments  are  purposely  made  witak.  the  result  of 
a  vessel's  striking  the  bridge  with  its  hull  will  Ik;  to  tear  tlieni 
away  and  swing  the  span  to  one  side.  Should  the  rigging  of 
the  vessel,  however,  strike  the  span,  theelleet  will  be  simply 
to  break  off  the  masts  without  injury  to  the  bridge.  This  lat- 
ter accident  Inis  happened  once  Jilrendy,  the  result  being  ex- 
actly what  the  author  had  prcdi<;ted.  There  is  a  spe(;ial  np- 
])aratus,  consisting  of  a  heavy  scpnire  timber  set  on  edge, 
trimmed  on  the  rear  to  lit  into  a  ,«(c(l  <  hannel  wliich  rivets  to 
tlie  cantilever-brackels  of  the  sidewalk,  and  fa<;ed  with  a 
6  X  0-in.  heavy  angle-iron,  to  act  as  a  cutting  edge.  This 
detail  is  a  very  ellecliveone  for  destroying  the  masts  and  rig- 
ging of  colliding  vessels. 

Tlie  bridge  is  designed  to  carry  a  double-track  street  rail 
way,  vehicles,  and  foot-passengers.  It  has  a  clear  roadway  of 
34  ft.  between  the  counterweight  guides  in  tin;  ti)wers,  the 
narrowest  part  of  the  structure,  and  two  cantilevered  side 
walks,  each  7  ft.  in  tlie  clear,  the  distance  between  central 
planes  of  trusses  being  40  ft.,  and  the  e.xtreme  width  of  sus- 
pended span  57  ft.,  except  at  the  end  panels,  where  it  is 
increased  gradually  to  (};{  ft.  The  roadway  is  covered  with  a 
wooden  block  pavement  34  ft.  wide  between  gUiiidiails  rest- 
ing on  a  4-in.  pine  floor,  that  in  turn  is  suppoiKMl  iiy  wooden 
«luiU8    which  are  bolted  to  15-iu.   I-beam   stringers,  spatted 


MOVAIM.Ii    HUIDGKS    IN    GENERAL. 


HI 


about  3  ft.  3  in.  from  cciiire  to  centre.  These  stringers  rivet 
up  to  the  webs  of  tlic  tioor-beams,  und  Ijcueuth  tbein  run 
{liiigonal  angles,  wliich  rivet  to  the  bottom  tlange  of  each 
stringer,  and  thus  form  a  very  eflicient  lower  lateral  system. 
'I'lie  sidewalks  are  covered  willi  2-in.  jnne  planks,  resting  on 
3  X  l!i-in.  pine  joists  spaced  about  2  ft.  from  centre  to  centre. 
Tlie  span  is  susjx  inled  at  each  of  the  four  upper  corners  of 
the  trusses  by  eight  steel  caldcs,  which  lake  bold  of  a  pin  by 
means  of  cast -steel  clamps.  Tliis  pin  passes  througii  two 
liangerplates  whicii  jiroject  abovr  the  truss,  and  are  riveted 
veiy  elloctively  to  tiie  end  post  by  means  of  the  portal  plale- 
ginler  strut  <m  the  inside  and  a  special,  short,  cantilever  girder 
on  the  outside. 

Each  portal-girder  carries  near  each  end  an  iron-bound  oak 
block  to  take  up  tlie  blow  from  the  hydraulic  bulfer,  Avhich 
hangs  from  the  overhead  girder  between  towers.  Sinular 
oak  blocks  are  let  in'o  and  project  from  the  copings  of  ti.e 
main  piers  to  take  up  the  blow  from  the  hydraulic  buffers 
that  are  attached  to  tiie  span. 

The  l)al last-tanks  before  alluded  to,  of  which  there  are  four 
in  all,  are  built  of  steel  plates  properly  stiffened,  and  have  a 
capacity  of  about  19,000  i)ouuds,  which  is  probably  more  than 
enough  lo  set  the  bridge  in  motion,  if  it  were  all  an  unbal- 
anced load.     These  tanks  serve  a  double  purpose,  the  lirst 
being  simply  to  balan(;e  the  bridge  when  it  gets  out  of  adjust- 
ment because  of  the  varying  load  of  moisture,  etc.,  on  the 
span,  and  the  second  being  lo  jirovide  a  quick  and  ellicient 
means  of  raising  and  lowering  the  span  in  case  of  a  to'al 
breakdown    of   the   machinery.     IF,  for  instance, — which   is 
inghly  improbable,  — the  operating   ropes   were   l)roken   and 
h;id  to  be  detached  from  their  drums,  by  emi.tying  all  of  the 
water  out  of  the  tanks  the  span  could  be  made  to  rise.     It 
(lould   be   lowered    again    by  tilling  them   from   a   reservoir 
which  is  placed  on  top  of  one  of  the  towers  and  kept  tilled 
wiiii  water  at  all  times  by  means  of  a  pum[)  in  the  machinery- 
house.     The  water  in  all  of  these  tanks  can  be  kept  from 
freezing,  or  the  ice  therein  can  be  thawed  at  any  time,  by 
turning  on  steam  from  the  machinery-room  into  the  coils  of 
pipe  whi{;h  they  contain. 

Tiie  oju'raling  machinery  is  located  in  a  room  37  X  53  ft,, 
the  opposite  si<ies  being  parallel,  but  the  adjacent  sides  being 
oblique  to  each  other,  the  cbiiipiily  amounting  to  about  13 
degrees.  The  placing  of  this  machinery  beneath  the  street 
was  really  forced  upon  the  author,  who  had  originally  con- 
templated using  eie(tii<;al  iniichinery  and  putting  it  in  a 
house  in  one  of  the  towers. 

The  arrangement  of  the  operating  machinery  is  as  follows  : 
Two  70-11.]*.   steam-engines  comnriuicjite  power  to  nn  8-h). 


112 


DE   PONTIBUS. 


horizontal  shaft  carrying  two  6-ft.  spiral-grooved,  cast-iron 
drums,  around  which  the  ^-in.  steel-wire  operating  cables 
pass.  As  one  of  the  lifting-ropes  passes  off  the  drum,  the 
correspoiKiing  loweriiig-rope  takes  its  place,  and  vice  versa, 
the  extreme  horizontal  travel  being  a  liltle  less  than  12  in. 
Thus  by  turning  the  drums  in  one  direelion  the  span  is 
raised,  and  by  turning  tliem  in  tlie  other  direction  ihe  coun- 
terweiglits  are  raised,  and  the  span  consequently  is  lowered. 
Wlii'n  the  span  is  at  its  lowest  position,  tlie  full  jwwer  of  one 
engine  can  be  turned  on  to  pull  up  on  the  counterweights, 
thus  throwing  some  dead  load  on  the  pedestals  of  the  spun, 
after  which  the  drums  can  be  locked.  Before  tlie  bridge  was 
completed  the  writer  considered  thtit  this  would  be  necessary, 
in  order  to  check  vibr.ition  from  rapidly  passing  vehicles;  but, 
sucij  has  not  proved  to  be  the  case,  for  tlu;  span  is  very  rigid, 
and  the  amoimt  of  the  vibration  is  not  worth  mentioning.  It 
is  possible,  tliougii,  that  in  some  other  lift-bridges,  where  the 
ratio  of  live  loud  to  dead  load  is  greater,  this  feature  of  opera 
tion  cannot  be  ignored. 

The  engines  are  provichjd  with  friction-brakes  that  are 
always  in  action,  e.\cei)t  when  the  throttle  is  opened  to  nioVc 
the  span  ;  consequently  no  unexpected  movement  of  the  span 
is  possible. 

The  raising-ropes,  after  leaving  the  drums,  pass  out  of  the 
machinery-house  to  and  beneath  some  5-ft.  idlers  under  the 
towers,  thence  up  to  the  top  of  Ihe  north  tower,  where  they 
pass  over  some  4-ft.  idlers  and  the  nuiin  12-ft.  sheaves.  Four 
of  them  here  pass  down  to  the  north  end  of  the  span,  and  the 
other  four  run  across  to  the  other  tower  over  more  idlens,  then 
down  to  the  south  end  of  the  span. 

The  loAcring  -  ropes,  after  leaving  the  drums  in  the 
machinery-room,  pass  under  some  idlers  below  the  north 
tower,  and  thciice  up  to  more  idlers  at  the  top  of  the  tower. 
Four  of  them  here  pass  down  to  the  counterweights  in  the 
north  tower,  and  the  other  four  run  across,  over  intermediate 
idlers  in  the  overhciid  bracing,  to  the  main  I'J-ft.  sheaves  of 
the  south  tower,  then  downward  to  the  counterweights. 

In  addition  to  the  previously  mentioned  method  of  moving 
the  span  by  the  water- ballast,  there  is  a  nnm-power  operating 
apparatus  of  simple  design  in  the  miichinery-house,  which, 
when  used  alone,  can  rai«e  and  lower  the  spjin  slowly  in  case 
the  steam-power  gives  out,  or  more  rapidly  when  combined 
with  the  water-ballast  method. 

As  the  span  ucars  its  highest  and  lowest  jmsitions,  an  auto- 
matic cut-off  apparatus  in  the  machinery-room  shuts  off  the 
steam  from  the  cylinders  and  thus  prevents  the  hydraulic 
builers  from  being  overtaxed. 


MOVAHLK    HKllKiKS    IN    (lENKKAL. 


113 


ADVANTAGKS  OF  LIFT-BRIDGES. 

The  ndvaiitiigos  of  lift-bridges  in  comparison  with  rotating 
drawbridges  are  as  follows  : 

1st.  A  lift-bridge  gives  one  wide  channel  for  vessels  instead 
of  the  two  narrow  ones  all'ordei'  by  a  centre-pivoted  swing- 
i)ridge. 

2d.  Tliere  are  no  land  damages  in  the  case  of  a  lift-bridge, 
MS  the  whole  siructiire  is  confined  to  tiie  widtli  of  tlie  street. 
These  land  damages  in  the  case  of  some  swing-bridges 
aniomit  to  a  large  percentage  of  tiie  total  osl  of  stiuctuie. 

;jd.  Vessels  can  lie  at  the  docks  close  to  a  lift-bridge,  which 
they  cannot  do  in  the  case  of  a  swing-bridge;  consequently 
with  the  former  tiie  dock-front  can  be  made  available  for  a 
mueh  greater  length  between  streets  than  it  can  with  tlie 
latter. 

4th.  The  time  of  operation  for  a  liftl)ridgc  is  about  30^ 
less  than  that  for  a  corresponding  swing-bridge. 

The  advantages  of  a  lift-bridge  in  comparison  with  a  bascule 
or  a  jack-kiufe  draw,  both  of  these  being  supposed  to  be  with- 
out a  centre  pier,  are  as  follows  : 

1st.  The  lift-bridge  can  be  made  of  any  desired  span,  while 
in  the  case  of  the  others  the  span  is  necessarily  quite  limited 
in  length. 

3d.  A  lift  biidge  can  be  paved,  while  the  others  cannot. 

3d.  The  lilt-bridge  is  very  much  more  rigid  than  any  struc- 
ture composed  of  two  or  more  pailially  or  wliolly  independent 
parts,  a  feature  characlerisiic  of  the  jack-knife  bridge  or  the 
bascule  without  a  centre  pier. 

4th.  In  a  lift-bridge  the  operating  machinery  is  much  more 
simple  ;  and,  in  case  that  it  should  ever  gel  out  of  order,  the 
span  can  be  raised  or  lowered  either  by  unbalancing,  or  by 
simple  hand  mechanism,  or  by  both  combined. 


If  the  author  were  to  design  another  lift-bridge  similar 
to  the  Halsted  Street  structure,  and  if  he  were  given  carte 
blanche  in  the  designing,  he  wouhl  make  the  following  im- 
provements : 

1.  Curve  the  rear  columns  and  iirch  the  overhead  girders  at 
tops  of  towers,  so  as  to  improve  the  general  appearance. 

2.  Operate  by  electricity  instead  of  by  steam. 

3.  Place  the  machinery-house  in  one  of  the  towers  and 
dispense  with  the  operating-house   on   the  span,  letting   the 


114 


1)K    I'ONTIHUS. 


operator  stiiml  in  a  bow -window  of  llie  maclunery-hotise  so  us 
1o  couiniiuul  11  view  of  llie  river  in  bulli  directions. 

4.  Omit  the  water  tanks  as  an  unnecessary  precaution,  and 
rely  on  the  great  capacity  of  llie  electric  motors  to  overconu' 
any  temporary  unbalanced  load. 

5.  A  simpler  and  less  expensive  adjustment  ut  feet  of  rear 
columns. 

6.  Cast  steel  instead  of  cast  iron  for  all  machinery. 

7.  Catch  the  balancing  chains  in  buckets  placed  on  top  of 
tiie  span  instead  of  hanging  them  to  the  coiuitcrweiglits. 

The  author  has  designed  a  r.illier  |)uculiar  lift-l)ridge  for  a 
crossing  of  the  Missouri  River  at  Kansas  Cily,  Mo.,  at  the 
site  of  the  unlinished  Winner  Bridge,  the  piers  for  which 
have  been  completed  for  over  six  years.  Tlie  proposed  super- 
structure will  provide  for  two  railway-tracks  on  eacli  deck,  a 
single-track  wagonway  outside  of  each  truss  below,  and  a 
foolwalk  outside  of  each  wagonway.  The  perpendicular 
distance  between  central  planes  of  trusses  is  to  be  thirty-two 
feet. 

The  requirements  of  navigation  will  be  provided  for  by 
means  of  a  lifting  deck  in  the  second  channel  span  from  the 
Kansas  City  side,  suspended  from  a  through  overhead  span. 
This  span  will  be  supported  on  steel  colunuis  carried  by  the 
existing  masonry  piers,  which  will  have  to  be  cut  down  to 
about  the  elevation  of  staiulard  high  water,  then  rebuilt  for 
two  or  three  courses.  At  one  end  of  the  supporting  span  the 
vertical  end  posts  are  made  fast  to  the  bent  posts  below  by 
means  of  a  pin  connection,  but  at  the  other  end  there  is  to  be 
a  nest  of  friction-rollers  between  the  foot  of  each  vertical  end 
post  and  the  lop  of  the  bent  colu  nn  benealii.  These  bents  are 
to  bo  stayed  to  the  inclined  end  post^  of  the  adjoining  spans. 

The  lifting  deck  will  consist  of  four  li;ies  of  railway  plate- 
girder  stringers  and  four  lines  of  open-webbed  highway 
stringers,  with  an  eifeclive  system  of  horizontal  and  vertical 
sway-bracing  between  liie  stringers  of  each  pa'r,  besides  a 
very  rigid  lateral  system  attached  to  the  lower  flanges  of  all 
stringers.  All  of  these  stringers  will  rivet  up  against  llu!  wel)s 
of  the  cross-girders,  the  elevulions  of  llie  upper  .surfaces  of  all 


MOVAULK    HKIDGKS    I X    (lENKKAI.. 


115 


loiigitudiiml  luul  cross  gilders  l)eiug  the  saino,  so  as  to  permit 
of  iimkiiig  the  longitudinal  girders  continuous  by  means  of 
coverplules.  To  permit  of  tljc  use  of  similar  cover-i)lates  for 
the  bottom  llanges  of  the  longitudinal  girders,  llie  webs  of  the 
cross-girders  are  to  be  slotted  for  their  passage,  and  the 
■weakened  web  sections  are  to  be  strengthened  by  means  of 
angle-irons. 

The  cross-girders,  wiiich  are  slightly  flsh-beilied,  are  to  be 
riveted  at  their  ends  into  hangers,  each  of  which  is  composed 
of  two  twelve-inch  I  beams,  the  distance  between  tho  vertical 
axes  of  hangers  being  forty-one  feet.  Beyond  the  bangers  will 
l)e  cantilever  brackets  for  carrying  the  highway  stringers, 
said  brackets  being  connected  at  top  to  the  cross-girders  by 
cover-plates  and  at  the  bottom  by  planed  ends  that  will  afford 
ellecllve  contact  for  the  meeting  flanges. 

At  the  top  of  each  hanger  is  a  detail  for  connecting  to  the 
cables,  and  beneath  the  same  is  placed  a  hydraulic  buffer  so 
arranged  tiial,  when  the  movable  deck  is  at  iis  lowest  position, 
the  live  load  thereon  is  carried  by  tiie  hangers  through  the 
buffers  to  certain  cantilever  brackets,  which  project  from  the 
ends  of  the  cross-girders  of  the  supporting  span. 

These  cantilever  brackets  and  the  tish-bellying  of  floor- 
beams  and  stringers  are  tho  only  peculiar  features  of  the  sup- 
porting span,  witli  the  exception  of  the  vertical  end  posts  and 
the  unusual  sizes  of  all  truss  members. 

While  the  live  load  of  the  movable  deck  is  carried  through 
the  hydraulic  buffers  to  the  bottom  of  tlie  supporting  span, 
the  dead  load  passes  by  means  of  the  wire  cables  to  the  top  of 
said  span. 

The  lifting  deck  will  be  operated  by  electrical  machinery 
located  in  the  house  at  the  middle  of  the  top  of  the  through 
or  supporting  span.  The  weight  of  the  lifting  deck,  which 
amounts  to  about  1,850,000  pounds,  is  counterbalanced  by  cast- 
iron  weights  iu  groups,  each  about  four  feet  long,  four  feet 
wide,  and  four  feet  six  inches  high,  strung  on  tightly  adjusted 
rods  to  hold  them  in  position  ;  and  is  supported  by  one  hun- 
dred and  twelve  steel-wire  cables  one  and  a  quarter  inches  in 
diameter  that  pass  over  flfty-six  cast-iron  sheaves  five  feet  in 


116 


UK    I'OXTIIU'.S. 


diaineler.  Tlitsc  slicaves  .'ire  conuecled  by  tnmsverse  three- 
inch  shufts  and  gearing  to  the  central  fuiir  and  a  halT  inc!i 
shaft,  which  runs  the  whole  leuglh  of  the  through  span  and 
connects  to  the  two  one  hundred  horse-power  electric  motors 
in  the  machinery  house.  Either  motor  alone  is  capable  of 
operating  the  lift  under  the  most  unfavorable  conditions.  Each 
sheave  supports  the  two  halves  of  a  wire  rope  about  one  huu 
dred  and  sixty-five  feet  long,  the  ends  being  run  into  sockets. 
This  rope  passes  around  a  twelve-inch  eipjalizing-wheel 
attached  to  the  counterweight  suspender,  so  as  to  adjust  any 
unequal  stretch  of  the  two  halves  of  the  rope. 

The  hydraulic  bulTers  previousl^^dcscribed,  thirty  in  num- 
ber, are  used  to  bring  the  deck  to  rest  at  the  lowest  position  of 
its  travel,  and  thirty  more  are  employed  for  the  .sau>e  purpose 
at  its  highest  position. 

In  addition  to  the  butlers  there  will  be  automatic,  electric 
cut-offs  to  remove  the  power  before  the  deck  reaches  either 
end  of  its  travel,  besides  powerful  brakes  to  bring  the  moving 
mass  to  rest  (juickly  whenever  the  operator  nuiy  so  desire,  atid 
always  automatically  at  the  higheftt  and  lowest  points  of  travel, 
in  order  to  relieve  the  bulTers. 

The  main  sheaves  are  five  feet  in  diameter  and  five  inches 
wide,  with  eight  radial  arms.  They  are  each  cast  in  one 
piece  and  keyed  to  a  seven-inch  steel  axle,  that  rests  on  two 
pillow-blocks  each  eight  inches  long,  fitted  with  bronze  bear- 
ings. 

The  pillow-blocks  rest  on  short  posts  riveted  into  long 
transverse  girders  that  rest  on  the  top  chords  and  cantilever 
out  beyond  them  about  tivn  feet  at  each  end.  These  posts 
are  to  be  well  braced  longiludinally.  The  supporting  detail 
between  the  transverse  girders  and  the  top  chords  is  such  as  to 
distribtite  the  load  properly  over  the  latter. 

There  will  be  at  each  of  the  four  corners  of  the  moving  deck 
two  rollers  for  transverse  motion  and  two  for  longitudinal 
motion,  all  acting  on  the  faces  of  the  columns  that  uphold  the 
supporting  span.  The  transverse  rollers  do  not  act  unless 
there  be  sufficient  wind-pressure  ou  the  deck  to  move  it 
laterally  ;  but  the  longitudinal  rollers  act  whenever  the  deek 


MOVAMLK    HKIDGES    IN    ORNEKAL. 


117 


is  moved,  iis  they  are  backed  by  springs  that  jiress  ihein  at  all 
times  ngainist  the  columns. 

When  the  dock  is  at  its  lowest  position  it  will  be  held 
firmly  to  the  piers,  with  n  proper  provision  for  longitudinal 
expansion,  in  such  a  manner  as  to  relieve  entirely  the  guide- 
rollers  from  carrying  the  wind-pressure,  so  that  they  can  act 
only  when  the  deck  is  raised. 

The  machinery-houso  will  l)e  about  twenty-two  feet  square 
and  fourteen  feet  high  under  the  eaves,  capped  by  a  dome, 
and  finished  in  an  ornamental  style.  The  door  is  to  be  of 
I  beams  supporting  a  four-incli  plank  floor. 

All  main  sheaves  are  to  be  covered  with  ornamental  hous- 
ings, and  all  gears  are  to  be  covered  with  small  galvani/.ed- 
iron  hinged  housings. 

The  velocity  of  the  lifting  deck  will  be  limited  to  one 
foot  per  second  by  means  of  an  autoniiilic  governor  attached 
to  the  electrical  machinery.  The  lime  required  lo  either  raise 
or  lower  the  deck  the  full  height  will  therefore  be  about  one 
minute. 

To  provide  for  a  possible  breakdown  of  the  electrical 
machinery,  a  man-power  apparatus  will  be  employed,  con- 
sisting of  two  capstans  connected  lo  the  main  shaft  by  means 
of  gearing  located  in  llie  m.nciiineryhouse,  and  operated  by 
levers  working  in  horizontal  pianos. 

The  moving  deck  and  counterweights  will  be  balanced  when 
the  deck  is  at  midlieight.  On  this  account  there  will  be  a 
constant  tendency  to  hold  the  deck  from  vertical  motion  at 
both  ends  of  its  travel,  because  of  the  unbalanced  weight  of 
the  wire  cables.  In  one  sens(!  this  will  be  a  decided  advan- 
tage, but  it  will  necessitate  extra  power  to  start  the  mass  in 
motion.  Again,  the  deck  will  be  balanced  for  ordinary  con- 
ditions of  weatlier,  but  il  is  probable  that  the  weight  will  be 
increased  by  moisture,  accumulated  dirt,  etc.  This,  if  it 
exist  to  a  moderate  extent,  will  be  an  advantage,  in  that  it 
will  tend  to  hold  down  the  deck  on  the  piers  ;  but,  as  before, 
it  will  require  increased  power  to  start  motion  and  to  operate. 
However,  the  amount  of  power  available  will  be  large  enough 
to  meet  all  conditions  of  loading  and  contingencies.     Should 


118 


r>K  ro  NTT  rims. 


tlie  deck  become  lighter  tlmn  the  counterwelphts  by  reason  of 
the  drying  of  tiie  timber  in  tiie  floor  uad  .screens,  it  will  1)0 
necessary  to  ndd  to  its  weight  by  louding  it  >  but  this  condi- 
tion is  not  likely  to  exist,  for  what  wciglit  is  lost  by  drying 
will  be  fully  nmde  tip  by  ftcciimnliited  dirt  in  spite  of  all  the 
l)r('C!»utions  that  may  be  taken  to  keep  the  Hoors  clean. 

Wliether  this  jiroposod  .structure  will  ever  be  built  is  pro  >- 
lemalical,  nltliough  there  is  a  fair  chance  of  its  being  finished 
some  day  with  modifications  tending  to  ciieapen  the  work. 
It  wojld  be  a  great  satisfaction  to  the  author  to  complete  this 
bridge  because  of  the  novel  design  for  the  lifting  deck. 

Floating  draws  are  a  type  of  structure  that  cannot  be  recom- 
mended except  as  a  temporary  expedient.  Tlie  author  liad 
occasion  once  to  design  one  of  them,  but  the  necessity  for  its 
use  did  not  develop,  so  it  was  not  built.  The  ol)jection8  to 
lloating  draws  are  as  follows  : 

1.  'I'rouble  from  rise  and  full  of  water,  necessiialinir  ((hi- 
slaiit  adjustments. 

2.  The  depression  of  tlie  dniw  under  the  live  load  and  llio 
consequent  changing  of  the  grmle. 

8.   Possible  disaster  from  injury  by  ice  or  drift. 

4.  Trouble  froiu  leakage. 

5.  Clumsiness  o*  method  of  opening  and  closing  the  draw. 
As  there  an;  nn  advantages  to  oflf.set  these  disiulvantages, 

unle.ss  it  be  possibly  a  small  .saving  in  first  cost  of  span,  it 
is  not  likely  thai  there  will  be  nuich  call  for  tioating  draws. 

In  concluding  this  chapter,  it  may  be  well  to  summarize 
somewhat  and  indicate  -Khnl  kinds  of  draws  should  be  used  at 
various  crossings. 

For  streams  bearing  a  moderate  amoiml  of  tnirtic  with  cross- 
ings located  in  country  districts  or  in  unimportant  cities, 
rotating  draws  are  tlie  cheapest  and  consecjuently  the  most 
appropriate  ;  but  for  great  trutttc  and  for  important  (titles 
bascule  and  lift  bridges  are  the  best, the  former  for  spans  up  to 
about  one  liundred  and  fifty  feet,  and  the  latter  for  longer 
spans.  The  choice  between  the  bascule  and  the  lift  for  all 
doubtful  cases  should  be  determined  simply  by  the  question 
of  first  cost. 


CHAPTER  X. 


RKVOLVIN'O    DUAWnUIDOEg. 


Revolvino  (Iniw-spans  nrc  required  when  bridges  across 
imvignble  Ktreaiiis  are  not  high  enough  above  tlie  water  to 
provide  tiie  proper  vertical  clearance  for  passing  vessels. 
Hefore  taking  up  the  discussion  of  draw-spans,  it  will  be  well 
to  consider  the  relative  advantages  and  disadvantages  of  high 
and  low  bridges  for  the  crossing  of  such  streams  as  the  Mis- 
sissippi, the  Missouri,  and  the  Arkansas  rivers. 

As  r  rule,  there  is  very  little  difference  in  the  first  cost  of 
a  high  and  of  a  low  bridge  for  such  a  crossing,  what  little 
there  is  being  in  favor  of  llie  latter  and  seldom  amounting 
to  more  than  ten  per  cent.  lOacli  pier  of  a  low  bridge  is 
(cheaper  than  the  corresponding  pii,;r  of  a  high  bridge  ;  but 
this  saving  is  offset  by  the  co«t  of  the  pivot-pier,  which  is 
extra.  The  superstructure  of  a  low  biidge  may  be  a  trifle 
lighter  than  that  of  the  corresponding  higii  bridge,  but  the 
more  expensive  metiil-work  of  the  draw-span  generally  over- 
balances this.  It  is  in  the  low,  short  trestle  approaches  that 
the  lov,  bridge  costs  less  than  the  high  one. 

As  these  approaches  are  generally  built  of  timber,  they 
have  to  be  renewed  about  once  in  every  eight  years,  and  the 
cost  of  renewal  is  a  regular  fi.Ked  charge,  which  lessens  the 
annual  net  income  from  the  bridge. 

Herein  lies  the  superiority  of  the  low  bridge  for  such  cross- 
ings.  Nor  is  this  its  only  advantage,  for,  by  its  adoption,  there 
is  generally  avoided  a  considerable  climb  at  each  end  of  the 
structure. 

On  the  other  hand,  the  low  bridge  involves  some  expense 
for  operation,  which  is  (piite  an  important  matter  when  there 
is  much  river  traffic,  but  which  is  of  slight  importance  when 

119 


120 


DE   POi»fTrBUS. 


the  draw  lias  to  be  opened  only  a  few  times  per  season,  us  is 
the  case  witli  bridges  over  most  Western  navigable  streams. 

Everytbiug  considered,  wbenever  tbere  is  any  cboice  be- 
tween a  bigh  and  a  low  bridge  for  tbe  crossing  of  any  impor- 
tant Western  river,  tbe  aulbor  favors  tbe  low  bridge,  not  so 
irucb  because  of  its  lower  first  cost,  but  on  account  of  tbe 
smaller  expense  for  maintenance. 

Tbe  different  kinds  of  revolving  draw-spans  recommended 
are  described  in  detail  in  Cbapters  XV  ami  XVII.  Tbey 
may  be  operated  in  various  ways,  for  instance  by  manpower, 
steam,  electricity,  gas  or  gasoline  engines,  or  water.  Wber- 
ever  an  unfailing  supply  of  electriciiy  is  available,  tbat  source 
of  power  is  tbe  best  and  cbea[)est.  Steam  is  appropriate  for 
large,  beavy  draws  wbcre  electricity  is  not  available.  Gas  or 
gasoline  engines  are  best  suited  for  comparatively  small  spans  in 
country  districts  ;  and  water-power  can  sometimes  be  employed 
to  advantage  where  tbere  is  a  fall  of  wat(!r  near  tbe  biidge. 

It  does  not  pay  to  use  storage  batteries  for  operating  draw- 
bridges. Concerning  tin's  questi(m  tbe  author  feels  that  he 
can  speak  as  an  aulliorily,  for  he  once  made  tbe  experiment, 
and  it  was  a  failure.  For  a  wliile  the  machinery  worked  to 
perfection,  but  soon  tbe  batteries  began  to  leak,  and  the  leak- 
age gradually  increased  to  such  an  extent  that  the  batteries 
would  not  hold  their  charge  for  three  consecutive  days  ;  so 
the  electrical  power  was  given  up,  and  the  bridge  has  since 
been  o^  erated  by  baud. 

Gasoline  engines,  everything  considered,  are  probably  the 
best  source  of  power  for  operating  the  average  draw-span. 
Tbe  author  has  lately  designed  some  small  draws  to  be  oper- 
ated therel)y;  but  the  maciiinery  has  not  yet  been  installed,  so 
be  cannot  report  concerning  bow  such  engines  act. 

In  respect  to  the  power  required  to  operate  draw-spans,  the 
author  uses  an  average  of  the  Boiler  formulie,  viz., 


H.  P.  = 


0.0125  TV  t> 
550       ' 


where  W=  total  load  on  rollers  in  pounds,  and  v  =  velocity 
on  pitch-circle  of  rack  in  feet  per  second. 


RKVOI-VINO    nRAWBUIDGES. 


121 


The  author  obtained  a  tine  cliecli  on  tlie  correctness  of  lliis 
formula  wheu  testing  tiic  draw-span  of  liis  Jefferson  City 
highway  bridge.  This  span  of  440'  weigiis  660,000  pounds, 
and  was  opened  by  four  jnen  in  four  minutes  and  tifty  sec- 
onds, Tiie  power  applied  by  the  men  was  measuied  by  dy- 
namometers, and  from  the  length  of  their  pith  and  from  their 
pull  the  horse  power  was  coni[)uted.  It  proved  to  be  just  a 
little  less  than  unity,  so  near  in  fact  that  it  was  called  unity. 
Tlie  velocity  v  was,  on  the  average,  0.066  feet  per  second. 
Substituting  in  the  formula  gives 

H.  P.  =  0.0125  X  660,000  X  0.066  ^  550  =  0.99. 


It  is  possible  that,  if  the  experiments  were  to  be  made  again, 
a  greater  divergence  from  the  formula  would  be  found,  for 
the  reason  that  the  bridge  is  liable  to  work  more  easily  after  it 
has  been  operated  a  while. 

The  computation  of  stresses  in  ordinary  diaw-spans  involves 
more  or  less  ambiguity.  The  assumptions  upon  which  the 
calculations  arc  biised  are  the  following  : 

!.  The  truss-rods  in  the  tower  are  so  light  that  they  cannot 
transfer  any  shear  pa^^t  the  pivot-pier;  consecjuently,  witii  a 
live  load  on  one  arm  oidy,  the  said  Jirm  acts  entirely  independ- 
ently of  tlie  other,  thus  making  tlx-  draw  for  tiiis  loading  con- 
sist of  two  simple  spans. 

2.  For  live  load  on  both  arms,  the  reactions  are  to  be  found 
on  the  a.ssumption  that  the  draw  is  a  continuous  girder  on 
four  points  of  support,  and  by  a  formula  based  upon  the 
'I'lieorem  of  the  Three  Moments  with  a  constant  monient  of 
inertia. 

Plate  IX  gives  a  diagram  from  which  can  be  read  at  a 
glance  the  percentage  values  of  the  reactions  for  any  balanced 
load  pln<ed  anywhere  on  any  span.  It  is  perhaps  theoretically 
not  (piite  perfect,  because  the  values  of  the  reactions  depend 
slightly  upon  the  ratio  of  distance  between  the  two  middle 
points  of  support  to  length  of  one  arm  ;  but  any  error  made 
by  assuming  this  ratio  as  constant  for  all   drawbridges  is  a 


DE   PONTIHITS. 


biigatelle    compared  with   t)ie    errors  ciiused   by   tbp    otlier 
assumptions. 

Candidly,  tbe  autlior  bns  very  little  faitb  in  even  tbe  ap- 
proximate correctness  of  tbe  orilinary  methods  of  computing 
live-load  stresses  in  draw-spans  ;  nor  has  be  mucb  more  in  tbe 
superrefined  metbods  involving  Ibe  principle  of  least  work, 
or  stretcbing  of  tbe  dilTerent  truss  members,  or  the  principle 
of  tbe  Three  Moments  with  varying  moments  of  ii  '^rti:  lu 
bis  opinion,  tiiere  is  but  one  satisfactory  method  of 
ing  the  reaclions  for  both  balanced  and  unbalanced  loatls, 
viz.,  by  making  large  models  of  a  number  of  sp:ins  of  various 
lengths,  and  weighing  therewilh  the  reactions  for  all  kinds  of 
loading.  From  a  series  of  experiments  of  this  kind  lliere 
could  be  prepared  a  diagram  or  diagrawis,  similar  to  tliat 
shown  on  Plate  IX,  which  would  give  approximately  correct 
reactions  for  all  spans  and  all  loa<lings.  Such  an  investigation 
would  rocpiire  con.siderable  time  and  money,  but  if  some 
professor  of  civil  engineering  would  undertake  to  make  tlu; 
experiments,  be  could  undoubteiUy  get  the  models  built  free 
of  charge  by  dividing  up  the  worit  among  several  of  tbe  lead 
ing  bridge-manufacturing  companies.  Tlie  results  of  such 
experiments  would  be  of  great  value  to  both  the  engineering 
profession  and  the  railroads  of  America. 

lu  fimiing  dead-load  stresses  in  draw-spans,  it  is  customary 
to  assume  that  tliL-  draw  is  open.  The  luthor  follows  this 
method,  but  also  assumes  an  upward  reaciioii  from  the  lifting 
machinery  at  the  ends,  and  tinds  tlu;  stresses  therefrom  ;  then, 
when  any  such  stress  tends  to  increase  the  section  of  any 
member,  it  is  considered,  but,  when  it  tends  to  decreas:;  the 
section,  it  is  ignored.  Tlds  method  certainly  is  liable  to 
involve  errors  on  the  side  of  safety  ;  but  tiie}  will  tend  to  oil" 
set  some  possible  errors  on  tbe  side  of  datiger  due  to  the  metht"! 
employed  in  lindmg  ll'e  live-load  stresses. 

There  will  be  no  attempt  made;  in  this  treatise  to  illustrai  ' 
lite  detailing  of  draw-spans  and  liieir  opeiating  machinery. 
There  is  a  little  work  on  "  Tlie  Designing  of  D«;'W-8pttns," 
by  Charles  II.  Wright,  C.K.,  wbicli  attcmpis  to  cover  this 
ground,  and  to  which  tin-  reader  is  n  fere  '  for  di'tailing  of 


IlEVOLVINO   DRAWnKTDnKS. 


123 


drawbridge  machinery.  Unfortunately,  though,  the  scales 
used  for  the  drawings  arc  generally  too  small  to  make  the 
illustrations  satisfactory. 

Although,  as  just  stated,  the  author  has  no  intention  of 
trying  to  cover  liere  the  subject  of  detniling  of  the  machinery, 
tliere  are  a  few  details  which  it  will  be  well  for  him  to  touch 
upon  iu  a  general  way,  among  others  the  question  of  rim- 
bcaring  rerxus  centre-bearing  turntables.  The  author  is 
deddedly  in  favor  of  the  former  because  of  the  greater 
stability  involved  when  the  load  is  carried  near  the  exterior 
of  the  pier.  Turntables  that  divide  the  load  between  the  rim 
and  c(!ntrc'  are  not  to  be  recommended,  because  the  division 
is  always  more  or  less  ambiguous.  The  load  should  always 
be  distribulod  as  uniformly  .„  possible  over  the  entire  drum 
and  among  the  rollers,  and  to  do  this  care  should  be  used  in 
designing  the  girders  over  the  drum  so  that  they  will  have  not 
only  the  necessary  strength,  but  also  the  proper  comparative 
rigidities,  'i'iie  greater  the  number  of  points  of  support  the 
more  evenly  will  the  load  be  distributed  to  the  drum  and  roll- 
<!rs  •  and  the  deeper  the  dnnn  the  better  the  distribution. 
Now  as  an  extra  foot  of  depth  of  drum  costs  much  less  than 
one  foot  of  height  of  pivot-pier,  it  stands  to  reason  that  it  is 
always  better,  whenever  i)nictirable,  to  make  the  drum  much 
dee|H'r  than  tin  ealeulations  for  strength  and  stiffness  demand. 
Tii(!  only  reason  for  not  adopting  in  every  case  an  excessive 
depth  is  that  so  doing  might  place  the  rollers  below  the  level 
of  high  water,  and  thus  render  the  span  liable  to  injury  from 
drift,  imd  the  niMchitiery  to  being  blocked  by  an  accumulation 
of  mud  under  and  between  tiie  wheels. 

\Viien  the  vertical  distance  between  high  water  and  the 
lowest  part  of  bottom  ^hord  is  small,  the  longitudinal  iind 
cross  girders  can  be  placed  with  their  bottom  Jianges  tlush  with 
tlie  lower  surface  of  the  bottom  chords,  and  the  drum  can  be 
built  inside  of  the  box  llius  formed,  so  that  its  bottom  Hange 
angle.)  shall  be  flush  with  the  bottoms  of  tiie  said  girder.s.  Or, 
if  the  vertical  clearance  be  great  enough  to  permit  it,  the  box 
may  rest  on  the  drum  at  eitiier  four  or  eigiit  points. 
As  a  rule,  drum  diameteis  are  made  loo  great  for  economy. 


124 


DE   PONTIBtJS. 


for  many  designers  think  it  necessary  to  rest  the  tower  posts 
directly  over  the  dnnn,  thus  inidiing  the  diameter  of  tiie  hitler 
about  forty  per  cent  grt-ater  tiian  the  side  of  the  square,  upon 
Ihe  corners  of  which  are  located  the  axes  of  tower  colunuis. 
Otiier  designers  make  the  sides  of  the  square  intersect  ihe 
t  of  the  drum  so  as  to  divide  the  latter  into  eight  equal 

pai '  thus  making  ihe  diameter  of  the  drum  about  eight  per 
cent  greater  than  the  side  of  the  square. 

Tiie  author  of  late  years  has  been  taking  the  diameter  of  the 
drum  equal  to  the  side  of  the  square,  and  has  obtained  eight 
points  of  support  by  inserting  four  small  girders  in  the  corners 
of  the  square,  at  angles  of  forty-five  degrees  with  its  sides. 
As  the  cost  of  a  pivot  pier  varies  very  nearly  as  the  square  of 
its  diameter,  it  follows  that  this  method  of  designing  the  drum 
effects  a  great  saving  in  cost  of  both  drum  and  pier.  Occa- 
sionally it  will  give  a  pier  of  very  small  diameter  in  com- 
parison with  the  length  of  the  draw-span.  The  remedy  for 
this,  provided  the  pier  have  the  requisite  stability  agrunst  over, 
turning,  is  not  to  increiise  the  jiier  diameter  but  to  anchor  the 
draw-span  to  the  pier  in  such  a  manner  as  not  to  interfere  with 
the  turning,  but  so  as  to  offer  an  effective  resistance  to  any 
tendency  to  lift  the  span  off  its  support. 

In  the  case  of  Ihe  Jefferson  City  liighvva}'  bridge,  tiie 
length  of  the  draw-span  is  four  hundred  and  forty  feet,  while 
the  diameter  of  the  drum  is  twenty-two  feet — the  same  as 
the  perpendicular  distance  between  central  planes  of  trusses. 
Such  a  ratio  of  span  length  to  drum  diameter  is  too  great  for 
safety  in  case  of  a  strong  lifting  wind  acting  on  one  arm  only, 
for  such  an  uplift  would  have  to  amount  to  only  twelve  and  a 
half  pounds  per  stpiare  foot  of  floor  in  order  to  throw  the  sp:in 
off  the  pier.  It  was  therefore  necessary  to  anchor  the  span 
to  the  pier  by  means  of  a  long  four-inch  bolt  passing  through 
a  wide,  heavy  casting  which  is  embedded  in  the  concrete,  and 
projecting  at  the  ui)per  end  between  two  beams  and  through 
a  saddle  and  a  heavy  washer-plate.  The  nut  on  the  anchor- 
bolt  is  turned  down  so  as  nearly  but  not  quite  to  touch  Ihe 
said  washer-plate,  thus  causing  no  obstruction  to  turning  the 


REVOLVING    DRAWBRIDGES. 


125 


(Imw,  but  mnking  the  anchorage  always  ready  to  resist  the 
slightest  tendency  to  lift  the  span. 

The  limiting  ratio  of  length  of  span  to  diameter  of  dnim 
that  can  be  employed  without  using  a  central  anchorage  cannot 
well  be  determined  by  ruk',  but  must  always  be  k-fl  to  ihe 
judgment  of  the  designer.  Il  might  suffice,  perhaps,  to 
specify  that,  wlienevor  the  uplift  on  one  arm  only  necessary 
to  upset  the  draw  is  less  than  twenty  pounds  per  square  fool 
of  tloor  in  situations  exposed  to  high  wind-pressiire,  or  less 
than  fifteen  pounds  in  other  situations,  an  anchorage  shall  be 
adopted. 

In  the  case  of  three  draw-spans,  which  the  author  has 
designed  lately  for  the  Kansas  City,  Pittsburg,  and  Gulf 
Railroad  Company,  the  span  length  is  two  hundred  and 
twenty-five  feet,  and  the  diameter  of  the  drum  is  only 
seventeen  feet ;  nevertheless  no  central  anciicrage  was  used. 
In  tiiese  bridges  the  open  fioor  reduces  the  uplift,  and  tlie 
situations  are  not  sucli  tliat  the  spans  will  be  exposed  to 
abnormally  high  wind-pressures. 

Heavy  draw-spans  should  be  operated  by  two  or  more 
piiuous,  and  when  these  are  placed,  as  they  should  be,  diago- 
nally opi)08ite  each  otlior,  some  kind  of  apparatus  ought  to  be 
used  to  equalize  the  pressure  on  the  pinions,  otherwise  both 
tlje  latter  and  the  rack  are  liable  to  have  tlieir  teeth  broken. 
The  reason  fortius  is  that  it  is  impossible  to  make  the  toothing 
of  the  rack  so  perfect  in  the  distance  of  the  semi-circumference 
that  opposite  pinions  operated  by  a  single  shaft  shall  at  all 
times  act  equally.  When  electrical  machinery  is  tised,  the 
equalizing  can  be  done  by  means  of  duplicate  motors  ;  but 
with  other  machinery  some  kind  of  mechanical  equalizer 
should  be  emplo^'ed.  The  author  several  years  ago  designed 
one  for  the  East  Onuiha  draw,  which  worked  to  perfection. 
It  was  made  by  cutting  the  engine-shaft  and  attaching  to  each 
end  a  bevel-gear  wheel.  These  bevel-gear  wheels  engage 
with  two  small  pinions  which  are  inserted  between  the  spokes 
of  a  large  spur-wheel  that  turns  loosely  on  the  engine-shaft. 
If  we  assume  the  pressures  on  tlie  main  rack-i)inions  on  each 
side  of  the  drum  to  be  constantly  equal  to  each  other,  the  two 


126 


DE    PONTIBUS. 


halves  of  the  eiigiiie-shafi  will  always  have  the  same  angular 
velocity  ;  but  in  case  the  pressure  on  the  teeth  of  the  two 
rack  pinions  on  one  side  of  the  drum  should  fall  below  that 
on  those  of  the  two  rack-pinions  on  the  other  side,  the  8i)ur- 
wheel  will  move  slightly  on  the  shaft  until  the  rack  pinions 
receive  L<iual  pressure  agniii.  By  this  apparatus  equal  pressure 
on  the  teitii  of  rack  and  pinions  is  at  all  times  insured.  The 
author  wus  convinced  of  the  necessity  for  such  a  device  by 
watching  it  when  the  span  was  being  turned  ;  for  several 
times  during  each  quarter  rotation  the  little  pinions  on  the 
spur-wheel  would  make  a  sudden  movement  of  such  magni- 
tude as  to  indicate  a  considerable  variation  in  the  spacing  of 
the  rack-teeth. 

In  designing  draw-spans  with  liigh  towers,  especially  Umg, 
double-track  ones,  there  is  an  important  matter  that  is  some 
times  overlooked,  viz.,  the  tendency  of  the  end  of  the  unloaded 
arm  to  rise  when  a  moving  load  is  on  the  other  arm.  For 
single-track  bridges  the  only  harm  that  this  would  do  would 
be  to  pound  the  end  bearings  ;  but  for  a  double-track  bridges 
it  would  certainly  some  time  cause  a  serious  disaster  by  the 
derailment  of  an  oncoming  train  when  the  other  track  on  the 
other  arm  is  covered  by  another  train.  Before  designing  the 
530-ft.  draw-span  for  the  East  Omaha  bridge,  I  he  author 
looked  up  this  matter  as  well  as  he  could,  having  heard  of 
tro\ible  being  experienced  from  rising  ends  on  a  double-track 
draw-span  but  little  shorter  than  the  one  then  contemplated. 
The  results  of  the  investigation  were  rather  contradictory,  so 
the  design  was  made  with  three  features  that  were  conducive 
to  resist  the  raising  of  the  ends,  viz.,  extra-deep  trusses  at  both 
inner  and  outer  hips  ;  stiff,  continuously  riveted  top  chords 
between  these  points  ;  and  an  end-lifting  apparatus  capable  of 
r.iising  the  ends  one  and  a  half  inches.  This  was  the  best  at 
that  time  which  the  author  could  do  to  avoid  the  diffl(;ully  ; 
l)ut  at  the  same  time  he  figured  upon  using  later  a  hohling- 
down  apparatus  in  case  the  neci  ssity  for  .same  should  ever 
arise.  As  explaine.l  in  Chapter  Xll,  this  span  has  at  present 
only  a  single  track  at  the  middle,  and  the  highway  cantilevered 
floors  are  not  yet  put  on      Observation  poves  that,  witli  one 


KEVOLVINO    DKAWBKIDOES. 


127 


arm  luuded  by  ii  tniiii  aud  the  oilier  arm  empty,  tliero  wus  no 
rising  of  Ihe  ends  when  the  latter  w(;ro  i)r()periy  supported. 
A  hite  inspection  showed  that  liie  timlier  criljs,  whidi  sire  used 
jis  a  temporary  support  for  tlie  ends  of  the  draw,  hnd  so 
shrunk  vertically  on  account  of  the  seasoning  of  the  timber, 
that  the  end  rollers  barely  touched  their  bearings,  so  the  latter 
will  have  to  be  shimmed  up.  This  condition  of  the  ends 
afTorded  an  excellent  opportunity  to  observe  the  rise  with  one 
arm  only  loaded  by  an  engine  and  enough  cars  to  cover  the 
said  arm.  The  amount  observed  was  three  eighths  of  an  inch. 
From  this  it  may  be  concluded  that  with  masonry  piers  and 
tlie  completed  superstructure,  and  with  a  hoist  of  one  and  a 
half  inches  by  the  lifting  gear,  there  is  no  chance  for  the  ends 
to  rise  from  their  bearings  ;  for,  to  cause  such  a  rise,  it  would 
take  a  live  load  just  four  times  as  large  as  the  test  load,  which 
is  more  than  could  be  placed  on  the  double-track  railway, 
wagonways,  and  footwalks.  Had  the  bridge  been  built  with 
shallow  trusses  and  with  eye-lmrs  in  a  portion  of  the  top 
chords  between  outer  aud  inner  hips,  as  was  the  similar  bridge 
which  was  reported  as  giving  trouble  from  rising  ends,  it  is 
probable  tlial  similar  difficulty  would  have  been  found  in  this 
structure. 

Home  engineers  may  think  that,  because  each  span  of  a  draw 
is  figured  as  an  independent  span  for  unbalanced  live  loads, 
on  the  assumption  that  the  longitudinal  tower  rods  are  so 
small  as  to  carry  no  vertical  shear  past  the  drum,  there  should 
be  no  tendency  for  the  end  of  one  arm  to  rise  when  the  other 
arm  is  loaded  ;  but  such  is  not  the  case,  as  the  tendency  would 
exist  if  there  were  no  l«)ngitudinal  tower  rods  at  all.  The 
rising  of  one  en. I  is  evidently  due  to  the  lowering  of  he  inner 
hip  of  the  other  span  aud  the  consequent  pull  of  the  inclined 
top-chord  eye-bars.  Now  imagine  two  cables  attached  to  the 
lop  of  the  tower  (which  is  still  assumed  to  be  without  longi- 
tudinal rods),  and  running  to  drums  on  the  shore.  When 
these  cables  are  strained  stifflcicutly,  the  far  end  of  the  draw 
will  rise  ;  and  under  these  conditions  there  will  be  no  vertical 
shear  whatsoever  in  the  tower  panel  of  the  truss.  As  far  a.s 
vertical  shear  in  this  paml  is  concerned,  the  coudilious  for  the 


138 


DB    I'ONTIBUS. 


case  of  the  strained  cables  and  for  the  single-loaded  arm  are 
identiciU,  hence  it  is  proved  thjit  one  end  of  a  draw  can  rise 
when  the  other  arm  is  loaded  and  when  the  longitudinal  tower 
bracing  is  incapable  of  carrying  any  verticid  shear  past  the 
pivot-pier. 

In  erecting  draw-spans,  some  method  of  adjustment  must  be 
provided  so  as  to  bring  the  ends  to  the  correct  elevation.  For 
comparatively  short,  spans,  say  up  to  200  or  even  250  feel, 
groups  of  thin  phites  on  top  of  piers  will  suttice,  as  the  grade 
can  be  adjusted  by  dapping  the  ties  or  joists ;  but  for  longer 
spans  there  will  be  needed  in  addition  to  this  method  an 
adjustment  in  each  of  the  bottom  chords  by  the  insertion  of 
several  thin  transverse  plates  in  the  panels  next  to  the  drum. 

The  tops  of  all  pivot-piers  slionhl  be  so  designed  as  to  drain 
thoroughly  by  pitchinir  the  ui)per  surface  from  the  centre 
towards  the  periphery,  an  1  by  providing  at  the  latter  weeping 
pipes  that  pass  below  the  lower-track  segments. 

In  large,  heavy  drawbridges  all  parts  of  the  turntable  and 
machinery  should  be  made  much  heavier  than  the  correspond 
iug  parts  for  smaller  structures,  even  if  there  be  no  theoretical 
reason  therefor  ;  because  the  tendency  in  the  past  has  been  to 
design  all  portions  of  the  machinery  for  the  exact  amounts  of 
work  that  they  are  assumed  to  do,  which  method  gives  for 
nnuiy  pieces  sizes  entirely  inadequate  for  some  conditions  of 
stress  to  which  they  are  likely  to  be  subjected.  The  propor- 
tioning of  turntables  and  machinery  for  draw-spans  is  a  matter 
involving  good  judgment  and  experience  in  operation  rather 
than  intricate  mathematical  calculations. 

The  authov  desires  to  call  attention  to  the  necessity  for 
making  all  man-power  machinery  extra  strong  ;  because,  if 
there  be  anything  wrong  with  the  apparatus  which  prevents  it 
from  operating  properly,  the  men  are  liable  to  crowd  upon  the 
levers  wherever  they  can  find  room,  and  then  surge  thereon  to 
their  utmost  capacity.  It  was  only  a  short  time  ago  that  in 
operating  the  East  Omaha  draw  by  hand  two  sets  of  six  or 
.seven  men  on  each  of  the  two  four-armed  levers  failed  to  start 
the  span  in  motion.  Immediately  upon  finding  the  unexpected 
resistance,  they  all  stepped  back  a  few  feet  and  threw  them* 


REVOLVING    DRAWMKIlXiKS. 


129 


selves  with  full  force  iipou  the  levers,  the  result  being  the 
same  as  before.  The  author  stopjjud  this  instiiiilly,  luul  upou 
investigation  found  tlmt  the  two  sets  of  men  were  woiking 
against  eacli  other,  so  by  stiirliiig  one  set  in  the  opposite 
direction  the  span  was  readily  put  in  motion.  This  example 
is  given  to  show  how  ignorant  workmen  will  abuse  machinery, 
and  the  consequent  necessity  for  making  nmn-power  apparatus 
e.\tra  strong,  notwithstanding  all  the  opposition  that  maybe 
offered  thereto  by  bridge  manufacturers.  It  is  thought  tliat 
the  method  of  proportioning  such  apparatus,  winch  is  specified 
in  Chapter  XV,  will  develop  ample  strength,  more  especially 
as  the  speciflcations  prohibit  the  use  of  cast-iron  gears. 

As  a  drawbridge  is  a  piece  of  machinery,  it  will  reiiuire  a 
certain  amoimt  of  care,  for  otherwise  it  will  get  out  of  order 
and  give  trouble  ju.st  at  the  wrong  time.  It  should  be  opened 
at  least  once  a  month,  and  all  parts  which  move  on  other  parts, 
especially  the  wheels  and  tracks,  should  be  kept  clean  and 
well  lubricated,  The  lower  rolling  surface  for  the  wheels 
should  be  kept  free  from  all  obstructions,  and  the  wheels 
should  be  maintained  in  proper  adjustment  by  means  of  the 
spider-rods.  Tiie  operating  machinery  also  should  receive 
due  care  and  attention. 


CHAPTER  XI. 


HlUnWAY   BRIDGES. 


SoMK  ten  years  ago  tlic  ftutlior  wrote  nn'l  publislied  n  pnin- 
plilct  entitled  "General  Specifications  for  Highway  Uridines 
of  Iron  and  Steel."  tiie  ()l)ject  of  wiiicli  was  to  affect  a  much- 
needed  improvement  in  tlio  designing  and  building  of  high- 
way bridges.  Through  the  Engineers'  Club  of  Kansas  City, 
•ftt  that  lime  a  flourishing  sxiiety,  h.it  now,  alas  1  (iefimct,  the 
pamphlet  was  placed  in  the  hands  of  a  great  number  of 
bridge  engiaecrs  Ihrouglioul  the  country,  wilh  n  request  Ihat 
they  discuss  it  for  pnblicatioM.  Many  of  tlieni  complied,  and 
their  discussions  were  published  in  tlie  Journal  of  the  Associ- 
ation of  Engineering  Societies  for  November  188H.  Soon 
afterwards  the  first  edition  of  tiie  pamphlet  was  exhausted,  so 
the  author  issued  a  second  edition,  revised  and  enlarged,  and 
incorporated  therein  most  of  the  said  discussions.  Now,  al- 
though both  editions  were  circulated  widely  among  county 
commissioners,  and  Hlthoiigli  the  author's  specifications  re- 
ceived the  general  indorsement  of  the  civil-engineering  pro- 
fession, the  effect  of  the  pam[)blet  on  the  methods  of  bridge- 
building  has  been  pradically  nil;  for  we  continue  to  read  in 
nearly  eveiy  issue  of  Enriineering  Neics  accounts  of  highway- 
bridge  failures,  many  of  them  accompanied  by  loss  of  life. 

In  truth,  the  number  of  iiigiiway  bridge  failures  is  on  the 
increase.  This  is  undoubtedly  partially  due  to  the  greater 
number  of  such  structures  in  existence  ;  but  it  is  also  due  con- 
siderably to  the  reckless  manner  in  which  highway  bridges 
continue  to  be  designc^l  a-ul  built,  owing  to  the  rapacity  of 
the  builders,  the  ignoranc;  and  dishonesty  of  the  commission. 
era,  and  the  lovv  moral  stale  into  which  tlie  designers  of  high- 

130 


HIGHWAY    BRinOKS. 


131 


way  bridges  liiive  fallen.  So  low  Is  that  state  that,  even  when 
given  III!  tlicf  metal  they  could  use  and  a  big  price  for  same,  it 
is  doubtful  whether  a  single  one  of  them  could  evolve  a  struc- 
ture scientiflcall}'  designed  throughout. 

Decidedly,  nothing  ran  be  done  for  liighway-bridge  build- 
ing through  county  ronnnissioners,  bc'vause  they  are  both  too 
ignorant  and  too  corrupt.  Notliing  but  tiie  strong  arm  of  the 
law  will  ever  reaeii  them  ;  and  the  only  way  to  force  them  to 
build  even  (le(;ently  strong  struf'turos  is  to  make  county  com- 
missioners criniinally  liable  for  all  injuries  to  persons  and 
pecuniarily  liable  for  all  injurien  to  proiMjrty  due- to  failures 
of  county  bridges  l)uilt  during  their  tenure  of  office.  Of 
course,  if  the  commissioners  could  prove  that  they  had  taken 
all  possible  precautions  by  having  the  structure  designed  by  a 
specialist  of  established  re|)utation.  built  by  a  good  manufac- 
turing comi>any,  and  inspected  by  first-class  inspectors  during 
both  manufacture  and  erection,  they  would  be  able  to  relieve 
themselves  from  the  responsibility  ;  but  if  they  were  to  do 
all  this  no  bridge  failure  (;ould  occur,  or  at  least  the  chances 
for  such  occurrence  woidd  be  extremely  small.  Some  such 
method  as  Ibis  for  placing  the  responsibility  for  bridge  dis- 
asters upon  county  commissioners  will  l)e  established  by  law 
some  day,  perhaps  in  the  not  very  distant  future  ;  and  the 
sooner  the  belter. 

As  matters  stand  now,  each  new  bridge  horror  stirs  up  the 
indignation  of  the  populace,  which  vows  thai  this  time  the 
guilty  parties  shall  be  brought  to  punisliment ;  but  the  inves- 
tigation generally  drags,  personal  intluence  is  broii^^  t  to  bear, 
njoney  is  often  used  judiciously,  and  the  result  is  that  nobody 
is  held  responsible,  and  the  disaster  is  soon  forgollen. 

If  each  stale  were  to  adopt  standard  speci(ic;itioiis  for  high- 
way bridges,  and  if  there  were  a  proper  officer  appointed  to 
see  thai  the  counties  live  up  lo  them,  much  good  would  be 
accom|)lished. 

The  second  edition  of  the  author's  pamplilet  on  highway 
briilgos  is  now  exhausted,  but  no  third  edition  will  be  issued, 
for  the  reason  that  le  jen  n'en  nmt  pan  la  chandelle.  There 
will  be  given,  however^  in  Chapters  XVI,  XVII,  and   XVIIJ 


13S 


I)E   I'ONTIBUS, 


of  this  book  complete  8|)ociti(:alioti8  for  the  (Icsigning  of  iiigb- 
•way  bridges  of  nil  isiuds  ;  consequeiitly  this  treatise  may  be 
said  to  replace  the  pamphlet  that  is  now  out  of  print. 

There  is  considerable  difference,  though,  belweeu  the  old 
speciticiitions  and  the  new,  due  to  two  reasons,  viz.,  first, 
there  have  been  great  advances  made  in  bridge-building  in 
the  last  eight  years  ;  and,  second,  the  author  .las  concluded 
to  abandon  the  attempt  to  conciliate  ,hose  who  desire  to  build 
cheap  structures,  so  has  cut  out  Class  I>  from  his  spccitlca- 
tions,  and  iias  strengthened  up  and  imi)roved  the  other 
"classes"  in  several  particulars,  notably  by  raising  the  mini- 
mum thickness  of  metal  from  one  (quarter  to  tive  'xteeulhs 
of  an  inch. 

The  weights  of  bridges  designed  according  new 

specifications  will  be  somewhat  greater  than  those  designed 
according  to  the  old  ones,  but  the  structures  will  be  corre- 
spoiulingly  better.  Moreover,  the  new  s|)ccilications  will  be 
found  to  be  more  rational,  scientific,  and  generally  satisfac- 
tory than  the  old  ones,  especially  in  the  feature  of  impact 
allowance.  It  must  be  remembered  that  the  author  does  not 
claim  that  the  formula  which  he  specifies  for  impact  will  pro- 
vide exactly  for  the  greatest  possible  impact  on  all  parts  of  all 
highway  bridges;  but  he  does  think  that  it  will  always  be 
great  enough,  and  he  knows  that  any  structure  in  the  design 
of  which  it  is  used,  and  which  is  proportioned  by  the  8|)ecifi- 
cations  of  this  treatise,  -will  be  well  and  properly  designed  in 
every  part. 


CHAPTER  xn. 


COMBINED  BRIDGES, 


As  a  rulo,  bridges  for  eiinying  Imth  rallwny  and  highway 
truffle  are  located  in  or  near  larg<  ities,  allhough  an  occa- 
sional structure  of  this  kind  is  found  in  country  districts. 
Tlie  princii)al  advantage  of  this  type  of  bridge  is  the  saving 
in  flrst  cost,  and  its  principal  disadvantage  is  a  reluctance 
to  cross  over  it  on  the  part  of  timid  drivers,  whose  horses 
may  be  frightened  by  the  trains. 

The  saving  in  first  cost  of  a  combined  railway  and  hfgliway 
bridge  as  ccmipared  ^\ith  two  separate  bridges  for  railway 
and  higiiway  traffic  is  considerable  ,  because  the  piers  for  the 
combined  bridge  are  but  little,  if  any,  more  expensive  than 
those  for  the  railway  bridge,  and  because  the  extra  metal  for 
the  superstructure  of  the  former  in  comparison  with  that  of 
the  latter  is  v"ry  much  less  in  weight  than  the  weight  of 
metal  required  for  a  separate  high  i 'ay  bridge. 

The  prejudice  against  combined  bridges  on  account  of 
danger  is  almost  wholly  unfounded,  for  horses  soon  become 
accustomed  to  railway  trains,  and,  when  screens  are  em- 
ployed to  hide  the  hitter,  but  little  trouble  is  experienced  on 
account  of  frightened  horses.  These  screens  may  be  made 
either  slatted  or  close,  the  former  offering  less  resistance  to 
the  wind,  and  the  latter  being  the  cheaper. 

The  advent  of  the  electric  railway  has  somewhat  compli- 
cated the  question  of  designing  combined  bridges,  for  now  it 
is  often  necessary  to  accommodate  three  or  four  kinds  of 
traffic,  viz.,  railway,  electric,  wagon,  and  pedestrian. 

When  a  highway  structure  has  to  carry  a  single-track  elec- 
tric line  in  addition  to    the   ordinary  highway  travel,  the 

188 


154 


DE   fONTlBt'S. 


author  classes  it  simply  as  u  highway  bridge,  for  it  is  seldom 
necessary  to  strengthen  it  inaterially,  because  of  the  electric- 
railway  load,  except  in  the  floor  and  primary  trass  nienibcrs. 
But  if  a  structure  lias  to  cany  either  a  single  or  double  truck 
electric  railroad  only,  the  author  treats  it  as  a  railroad  bridge. 
Combined  bridges  may  be  divided  into  tlie  following 
classes  : 

1.  Structures  having  a  single  deck  for  all  kinds  of  tratlic, 
the  railway  occupying  the  centre  of  the  bridge,  and  the  elec- 
tric railway  lying  close  to  one  truss. 

2.  Structures  having  a  single-track  railway  at  the  middle, 
a  narrow  footwalk  on  each  side  of  same  inside  of  the  trusses, 
and  cantilever  brackets  outside  of  the  latter  to  carr}'  wagon- 
ways  and  electric  lines.  Tiiis  arrangement  may  be  varied  by 
running  the  electric  cars  over  the  main  railway  track,  tlnis 
leaving  the  wings  free  for  wagou  traffic. 

3.  Structures  liaving  a  double-track  railway  inside  of  the 
trusses,  with  long  cantilever  brackets  outside  carrying  wjigons 
and  electric  lines  next  to  the  trusses,  and  pedestrians  outside. 
This  arrangement  may  be  varied,  as  in  Case  2,  by  carrying  the 
electric  trains  on  either  one  or  both  o'  the  main  raihviiy  tracks. 

4.  Structures  having  a  double-track  railway  inside  of  the 
trus.scs,  with  short,  cantilever  brackets  for  wagon  and  electric- 
railway  traffic  outside,  and  either  a  single  passagewjiy  over- 
head at  the  middle  for  pedestrians,  or  two  passageways  for 
same  on  overhead  brackets  outside  of  the  trusses.  As  before, 
this  arrangement  may  be  modified  by  running  the  electric 
trains  over  the  main  railway  tracks. 

5.  Double-deck  structures  carrying  railway  trains  on  one 
deck  and  wagons,  electric  trains,  and  pedestrians  on  the  otlier. 
The  pedestrians  may  be  accommodated  either  inside  tiie 
trus-ses  or,  preferably,  by  exterior  walks  on  cantilever  brack- 
ets. The  railway  may  be  placed  either  above  or  below  to 
suit  the  existing  conditions,  or  the.ro  mny  be  cither  a  single- 
track  or  a  double-track  railway  both  above  and  below,  with 
wagon-ways  and  pcdestrian-wavs  outside  of  the  trusses  cither 
above  or  below. 

Class  No.  1  is  the  cheapest  possible  kind  of  combined  bridge. 


COMBINED   BRIDGES. 


135 


and  at  the  same  lime  ihe  most  unsatisfactory,  for  wlieu  h 
railroad  train  is  about  to  pass  over  the  bridge  ail  wagon  and 
electric-railway  tnivel  must  be  kept  off,  and  because  pedes- 
trians must  look  oKt  sharply  for  their  safety  when  ou  the 
structure  with  a  railwa}'  train  crossing.  Their  danger  is 
really  greater,  though,  when  jin  (tlcclric  train  is  passing  a  team 
or  teams.  The  least  allowable  clear  width  of  bridge  for  this 
class  of  structure  is  twenty  feet,  the  electric  cars  running  on 
a  third  rail  and  on  one  of  the  rails  of  the  main  railway.  The 
author  has  built  a  large  bridge  of  this  class,  and  it  has  never 
given  any  trouble  from  the  combined  tralHc,  which,  however, 
is,  up  to  the  present,  rather  light. 

(.Mass  No.  2  is  a  very  satisfactory  type  of  structure.  The 
atithor  has  designed  and  built  several  bridges  of  this  kiiul,  the 
largest  of  which  is  'he  (combination  Bridge  ('ompany's  bridge 
over  the  Mi«so\iri  River  at  Sioux  City,  Iowa.  It  consists  of 
two  draw-spans  of  470  feet  each  and  two  fixed  spans  of  500 
feet  each,  the  distance  between  central  planes  of  trusses  being 
twenty  live  feet. 

Class  No.  3  is  also  a  satisfactory  type  of  structure.  The 
author  has  built  a  large  bridge  of  this  class,  viz.,  the  one 
across  the  Missouri  River  at  East  Omalia,  Nebraska.  This 
class  of  stru(;ture  involves  very  heavy  metal-work  ;  but  it  is 
not  une(!onomical. 

Class  No.  4  is  an  unusiial  type,  and  is  not  likely  to  be  called 
for  very  often,  although  the  author  once  had  o(;casion  to  figure 
on  a  bridge  of  this  kind. 

Class  No.  5  is  a  very  good  comi»ination  that  can  be  modified 
to  suit  nearly  any  conditions  of  »  ombincd  trallic. 

A  good  example  of  this  is  the  design  described  in  Chapter 
IX  for  the  Kansas  City  and  Atlantic  Railway  C-'ompany's 
proposed  bridge  over  th(;  Missouri  liiver  at  Kansas  City,  Mo. 

The  East  Omaha  Bridge  just  referred  to  ailords  an  excellent 
exaniple  of  how  to  keep  down  tiie  first  cost  of  a  structure  and 
yet  build  it  .so  that  it  can  be  enlarged  later  after  the  bu-niness 
devel()i)s.  The  design  for  th(;  linal  structure  involves  a  draw- 
spun  of  MO  feet  and  a  fixed  spun  of  5(50  feet,  carrying  ii  double 
track  railway  between  the  trusses,  a     Mubined  wagonwuyaud 


136 


DE   PONTIBUS. 


electric  railway  outside  of  facU  truss,  aud  a  pedestrian-Way 
outside  of  eucli  wagou-way,  the  bridge  crossing  the  river  at 
right  angles  ;  while  tlie  present  structure  consists  of  the  520- 
foot  draw-span,  without  the  wings,  aud  three  single-track 
combination  spsins  of  192  feet  each,  all  the  piers  except  the 
pivot-pier  being  built  of  piles  and  timber,  and  the  centre  line 
of  structure  making  an  angle  of  eleven  degrees  with  the  centre 
line  of  the  final  bridge.  The  deck  carries  a  single  railway 
track  ill  the  middle  and  an  electric  line  by  means  of  a  third 
rail  to  one  side.  All  four  classes  of  travel  U3e  this  deck.  The 
only  portio;!  of  the  existing  structure  that  is  really  finished 
is  tlie  pivol-pic",  which  consists  of  a  double  steel  cylinder 
forty  feet  in  diameter  sunk  by  open  dredging  to  bed-rock, 
wliicli  lies  one  hundred  and  twenty-two  feet  below  extreme 
low  water.  The  completion  of  the  draw-span  will  be  a  very 
simple  matter,  consisting  merely  of  adding  the  cantilever 
brackets  with  tlieir  stringers  and  flooring  and  laying  the 
electric-railway  rails  thereoti.  The  remaining  jners  for  the 
final  strucliue  can  all  be  put  in,  and  the  fixed  span  can  then 
be  placed  on  them  without  interrupting  trattic,  because  of  the 
deflection  downstream  of  the  present  temporary  structure. 
When  the  new  biidge  is  completed,  all  that  it  will  be  necessary 
to  do  is  to  rotate  the  draw  eleven  degrees,  so  that  the  traflic 
iiuiy  be  Iransferrod  thereto.  Afterwards  the  old  pile  piers  and 
tje  combination  spans  can  be  removed  at  pleasure. 

In  designing  combined  bridges  of  all  clas.ses  except  No.  1, 
a  considerable  economy  of  metal  may  be  elfectei'  egitimately 
by  keeping  tiie  total  live  load  as  low  as  is  proper  with  reference 
to  the  theory  of  probabilities.  For  instance,  in  Class  No.  2 
the  live  load  for  trusses  can  be  determined  by  adding  to  the 
equivalent  uniform  live  load,  given  in  the  diagram  ou  Plate 
III  or  Plate  IV,  a  much  smaller  highway  floor  load  per  lineal 
foot  of  span  than  that  prescribed  in  the  specificationg  for 
highway  bridges,  because  when  the  greatest  train  load  is  ou 
the  bridge,  the  chances  of  having  a  heavy  highway  live  load 
are  very  small.  The  longer  the  span  the  smaller  may  the  live 
load  per  scpiare  foot  of  floor  be  taken  when  finding  the  total 
live  load  for  the  trusses. 


t!i 
tl 

li 

bi 
til 
hi 
n( 
m 
fo 
li 


COMBINED    l{  HI  DOES. 


Vdl 


Again,  in  Cliisses  No.  3  and  No.  4  it  would  be  legitimate  to 
take  liie  live  load  per  lineal  foot  for  the  raihva}'  equal  to  twice 
the  car  load  per  liueal  foot,  and  add  thereto  a  small  highway 
live  load  as  in  the  last  case. 

Finally,  in  Class  No,  5  it  would  be  proper  for  a  four-track 
bridge  to  make  the  live  load  for  the  tru.s.ses  equal  to  four 
times  the  car  load  per  lineal  foot,  and  ignore  entirely  the 
liighway  live  load  ;  for  the  greatest  combined  live  load  would 
never  amount  to  four  times  the  tar  load.  This  was  the 
metliod  pursued  by  the  author  in  determining  tiie  live  load 
for  the  trusses  of  I  lie  proposed  Kansas  City  and  Atlantic 
Hallway  Company's  bridge  referred  to  iu  Chapter  IX. 


CHAPTER  XITI. 


DETAILING, 


It  is  ouly  within  h  few  years  that  much  attention  has  been 
given  to  detailing  by  bridge  engineers,  tlie  old  custom  having 
been  for  the  engineer  to  figure  the  diagram  of  stresses,  or,  as 
it  was  llien  called,  tlie  strain-sheet,  and  pass  it  over  to  a  drafts- 
man (too  often  a  cheap  one)  to  make  tlierefrom  the  working 
drawings  of  the  bridge,  using  probably  some  old  drawings  of 
another  l)ri(lge  as  a  guide  for  the  detailing.  Concerning  tlie 
evil  eflfects  of  such  a  course  of  action  the  engineer  who  docs 
much  inspeci  ion  of  existing  structurescan  speak  authoritatively. 
If  questioned  upon  the  subject,  any  such  engineer  will  .say 
that  nearly  all  l)ridges  which  fail  or  which  are  condemned  and 
removed,  are  deficient  in  strength  of  details  rather  than  in 
strength  of  main  members. 

Some  ytniva  ai;'>  the  author  had  t)crasion  to  examine  and 
report  upon  nearly  all  the  bridges  on  two  hundred  miles  of  the 
main  line  of  an  iin|)ortant  Western  road,  with  the  rcsull  that 
he  found  it  necessary  to  condemn  almost  all  of  them.  A  few 
have  since  been  repaired,  l)ut  most  of  theni  have  been  taken 
out  and  replaced.  In  mo.st  of  these  condemned  structures  the 
detailing  was  so  faulty  that  the  bridges  were  gradually  racking 
to  pieces,  and  no  amount  of  patchwork  wotild  have  made 
them  really  serviceiblc.  It  is  true  tluit  the  main  members  were 
considerably  overstrained  by  reason  of  the  increase  in  rolling 
load.s,  but  had  the  details  be<'n  first-class  the  structures  would 
have  been  standing  today. 

Jiist  here  it  may  be  w  U  to  mention  that  the  inspection  of 
these  biidges  caused  the  author  to  establish  for  himself  the 

188 


r)ETAILII^G. 


139 


following  priuciple.  which,  ns  it  does  not  pertain  to  bridge- 
designing,  is  not  given  in  Clmpter  II :  "  lu  nine  cases  out  of 
ten  tlie  proper  wuy  to  strengthen  a  weak  bridge  is  to  take  it 
out  and  replace  it  with  a  good  one,  throwing  the  old  metal 
into  the  scrap  heap." 

In  railway-l)ri(lge  designing,  for  a  number  of  years,  the 
average  ratio  of  weiglit  of  details  to  weight  of  main  members 
has  been  gradually  increasing;  and  the  end  is  not  yet,  because 
the  average  l)ridgedeHigner  has  still  a  great  deal  to  learn  con- 
ceridng  the  importance  of  good  and  efflcient  detailing.  As 
long  as  contracts  for  bridges  are  awarded  to  bridge  companies 
on  competitive  designs,  anti  the  structures  are  paid  for  by  tlie 
lump  sum  instead  of  by  the  pound,  just  so  long  will  the 
science  of  detailing  be  ignored,  and  just  so  long  will  bridges 
be  built  which  will  eventually  wear  out,  simply  for  want  of  a 
little  more  metal  distributed  just  where  it  is  needed,  viz.,  in 
the  details. 

The  author  feels  that  lie  cannot  speak  too  forcibly  concern- 
ing the  importance  of  thoroughly  scientitic  detailing  for  all 
kinds  of  metal-work;  for  what  avails  it  that  a  st'-ucture  have  an 
e.\(!ess  of  section  in  every  main  member,  if  a  single  important 
detail  be  lacking  in  strength?  If  the  author  were  in  a  position 
where  he  had  to  cut  down  the  weight  of  a  structure  even  as 
much  as  thirty  y.'r  cent,  he  would  unhesitatingly  take  the 
metal  almost  entirely  out  of  tlie  sections  of  the  main  members 
and  leave  the  detailing  practically  unchanged.  A  structure 
thus  designed  would  long  outlast  one  of  the  same  type  in 
which  the  weiglit  of  the  details  and  that  of  the  main  members 
were  reduced  in  the  same  proportion. 

A  few  years  ago  the  standard  text  books  on  bridges  ignored 
entirely  the  subject  of  detailing.  Later  they  have  taken  cog- 
nizance of  it  by  illustrating  certain  details  in  common  use,  both 
good  and  bad  (generally  the  latter),  but  have  failed  to  state  the 
fundamental  principles  that  sho(dd  govern  the  designing  of  all 
details.  These  general  underlying  principles  and  complete 
instructions  as  to  how  to  detail  scientifically  the  author  has 
endeavored  to  give  in  Chapters  II,  XIV,  XV,  XVI,  and  XVII 
of  this  wo-  k.    The  bridge  designer,  by  studying  these  chapters 


140 


DE    PONTIBUS. 


carefully,  mastering  all  of  their  contents,  and,  while  making 
his  drawings,  applying  the  principles  therein  given,  will  be 
able  to  evolve  structures  that,  to  say  the  least,  will  be  a  great 
improvement  on  the  average  structure  in  common  use. 


CHAPTER  XIV. 

GENERAL  SPECIFICATIONS  GOVERNING  THE  DESIGNING  OF 
STEEL  RAILROAD  BRIIKIFS  AND  VIADUCTS  AND  THE 
SUPERSTRUCTURE  OF  ELEVATED   RAILROADS. 

GEXEBAL    DESCBIPTIOi^ 

MATEniALS. 

All  parts  of  the  structure,  except  ties,  foot-plauks,  and 
guard-timbers,  sliall,  for  all  spans  of  ordinary  lengtlis,  be  of 
mediuin  steel,  excepting  only  that  rivets  and  bolts  are  to  be  of 
soft  steel,  and  adjustable  members  of  either  soft  steel  or 
wrought  iron.  For  very  long  spans  high  steel  may  be  used 
for  top  chords,  inclined  end  posts,  pins,  eye-baia  in  bottom 
chords,  and  those  in  mai!>  dingonals  of  panels  where  there  is 
no  reversion  of  stress  when  impa::t  is  included.  It  may  be 
used  also  for  the  web-members  of  cantilever  and  anchor  arms 
in  cantilever  bridges  where  the  variation  of  stress  is  com- 
paratively small  and  where  the  impact  cannot  be  great.  E.\- 
cepting  for  purely  ornamental  work,  cast  iron  will  not  be 
allowed  to  be  used  in  the  siiperstructure  of  any  bridge, 
trestle,  or  elevated  railroad,  cast  steel  being  employed 
wherever  important  castings  are  necessary. 


CU08H-TIE8,    FOOT-PLANKS,    AND   GUARD-TIMBERS. 

Cross-ties,  foot-planks,  and  guard-timbers  shall  be  of  long- 
leaf.  Southern  yellow  pine  or  other  timber  whicli,  in  the 
opinion  of  the  Engineer,  is  equally  good  and  serviceable. 
The  wooden  tloor  shall  be  so  designed  as  to  ensure  safety  from 
passing  trains  for  the  ndlroad  employees.  The  spaces  between 
ties  sliall,  in  general,  not  be  less  than  five  (^))  inches  nor  more 
than  six  (6)  inchps  wide.     The  sizes  pf  ties  shall  be  such  09 

lit 


142 


DE    PON TI BUS. 


to  give  the  requisite  resistftnce  to  bending,  under  tlie  assump- 
tion thai  the  load  on  one  pair  of  wheels  is  distributed  equally 
over  three  ties,  the  cfTect  of  impact  being  considered.  No  tie 
shall  be  less  than  seven  (7)  or  preferably  eight  (8)  inches  wide, 
nor  less  th!U\  six  (6)  inrlies  deep,  nor  less  than  ten  (10)  feet 
long,  e.\ee])t  in  the  ea^eof  elevated  railroads,  wlu're  llie  Icnglli 
may  be  rediieed  to  eignt  (8)  feet  for  a  spacing  of  live  (5)  feel 
between  central  planes  of  longiludinal  gliders. 

Ties  shall  bedapjied  to  a  full  and  even  Ixaring  not  less  than 
one-half  (A)  inch  onto  Ihe  stringers  ;  and  each  aherna'e  tic; 
siiali  be  secured  thereto  at  each  end  by  a  three-quailer  (Jj  inch 
hook  bolt. 

All  timber  bolts  shall  be  of  soft  steel,  with  cold-pressed 
threads. 

Outer  guard-timbers  shall  be  6 '  X  H"  laid  on  tiat,  dapped 
one  (1)  inch  onto  the  ties,  and  placed  so  thtit  their  inner  faces 
shall  be  just  twelve  (13)  inches  from  the  gauge  planes  of  rails. 

Wiiere  inner  guard-timbers  are  em])loyed,  they  shall  be 
6"  X  8"  on  flat,  dapped  one  (1)  inch  onto  the  ties,  and  place.l  so 
that  their  outi  ^aces  shall  be  just  five  (5)  inches  from  the 
gauge-planes  of  rails. 

Each  guard-rail  must  be  bolted  to  each  alternate  tie  by  a 
three-quarter  (J)  inch  .screw-bolt,  the  head  of  whieii  shall  be 
countersunk  into  the  wood  by  means  of  a  eup-siiaped  washer. 
Each  guard- limber  nui.st  be  spliced  over  a  tie  with  a  half-.-ind- 
lialf  joint  of  at  least  si.\  (0)  inches  lap,  througii  which  must 
pass  a  three-quarter  (|)  inch  screw-bolt. 

Guard-timbers  shall  e.\tend  over  all  piers  and  abutments. 

Steel  rails  or  heavy  steel  angles  well  fastened  to  the  ties  may 
be  substituted  for  the  inner  wooden  guard-rails,  or  the  inner 
guards  may  be  omitted  allogelher  if  the  Engineer  so  direct. 


IlEUAIMNQ   ArrAHATUS. 

At  each  end  of  every  bridge  or  trestle,  there  is  to  be  placed  a 
rerailing  apparatus  that  will,  in  the  n)osl  etlective  manner 
practicable,  retiirn  to  the  track  any  derailed  car  or  locomotive 
that  is  not  more  than  half  the  wjdth  of  track  gauge  out  of 
line. 


GENERAL    DESCRIPTION. 


143 


BUCKLED-PLATE   FLOOHS. 

If  the  Engineer  so  desire,  a  bucklcd-plute  floor  with  ties  in 
biillast  may  be  used  iusteiid  of  the  wooileu  floor,  in  whieh  case 
llie  size  of  the  ties  may  be  reduced  to  6"  X  8"  X  8'. 

All  buckled  plate  floors  must  be  thoroughly  diaiued  so  as 
not  to  retain  water,  and  the  upjier  surface  of  the  buckled  plate 
nuist  be  protected  from  rusling  by  u  liberal  use  of  the  best 
obtainable  preservative  coating. 


SUVEUELEVATION  ON   CUKVES. 

On  curves  the  outer  rail  will  be  elevated  the  proper  amount 
for  the  degree  of  curvature  and  for  the  assumeil  medium 
velocity  of  tnuns  ;  and  this  elevation  must  be  framed  into  ties, 
as  no  shims  will  be  allowable  anywhere  under  ties  or  rails,  ex- 
cepting ill  the  case  of  very  sharp  curves  requiring  a  superele- 
vation exceeding  three  (S)  inches  in  five  (5)  feet,  on  wliich 
long  shimming  timbers  are  to  be  bolted  to  the  top  flanges  of 
the  outer  longitudinal  girders,  or  short,  substantial  ones  to 
tops  of  ties,  so  as  to  give  the  reciuired  superelevation. 

The  formula  to  be  used  for  total  superelevation  on  standard- 
gauge  roads  is 

0.3277  7'^ 


E= 


U 


where  E  is  the  total  snpereh,'vation  in  feet  of  the  exterior  rail 
above  the  interior  rail,  V  is  tiie  assumed  velocity  of  train  in 
miles  per  hour,  and  Ji  is  the  radius  of  the  curve  in  feet.  The 
total  superelevation  is  to  be  obtained  by  depressing  the  inner 
rail  and  elevating  the  outer  one  equal  amounts,  thus  preserving 
the  grade  of  the  centre  line. 

SPACING  OF   STIUNGEU8,    GIRUKRS,    AND  TllACKS. 

In  general,  stringers  for  through  bridges  shall  be  spaced 
eight  (8)  feet  centres  for  single  track  bridges  and  six  (6)  feet 
six  (6)  inches  for  double-track  bridges  and  half-through  plate- 
girder  bridges.  In  elevated  railroads  the  spacing  of  the  longi- 
tudinal girders  may  be  made  as  small  as  five  (5)  feet  centres. 

Peck  plate-girders  may  be  spaced  from  six  (6)  feet  to  ten  (10) 


144 


DE    PONTIKUS. 


feet  centres,  the  usual  distiince  being  the  nearest  even  foot  to 
one  tenth  (jio)  of  the  span;  but  in  high  trestles  the  spacing 
shall,  preferably,  be  ten  (10)  feet,  and  never  less  than  eight  (8) 

feet. 

The  standard  distance  between  centres  of  Imcks  on  tangent 
for  surface  railroads  shall  be  thirteen  (13)  feet,  while  for  ele- 
vnled  railroads  it  shall  generally  be  twelve  (12)  feet. 


gPACINO   OF   TRUSSES. 

From  centre  to  centre  of  through  trusses  \hv.  perpendicular 
distance  shall  not  bo  less  than  seventeen  (17)  feet,  or  one 
twentielli  (gV)  of  the  span  length. 

From  centre  to  centre  of  deck,  i>in-connected,  or  riveled 
tnisses  the  perpendicular  distance  shall  not  be  less  than  leu 
(10)  feet  or  one   thir- 
teenth ( i\)  of  the  span  |<^       i  0       ^ 
length,  except  in   the 
case  of  elevated  rail- 
roads,    where    open- 
webbed,   riveled    gir- 
ders     are      adopted. 
These  may  be  spaced 
according  to  the  direc- 
tions given  for  plate 
girders. 

CLEARANCES. 

The  clear  opening 
on  tangent  shall  not  be 
less  than  that  shown 
in  Fig.  7. 

On  curved 
track,  the 
horizontal 

distance  from  the  centre  of  track  to  clearance  line  shall  be 
increased  at  all  points  two  (2)  inches  for  each  degree  of 
curvature. 


BASE   or  RAIL 


Fio.  7. 


GENERAL   DESCKU'TIOX. 


145 


EFFKCTIVB   LENGTHS. 

Effective  lengths  shall  be  as  follows  : 

For  pin-coiinected  spans,  the  effective  length  shall  be  the 
distance  between  centres  of  end-pins  of  trusses. 

For  riveted  girders,  it  shall  be  the  distance  between  centres 
of  bearing-plates. 

For  stringers,  it  shall  be  the  distance  between  centres  of 
cross-girder  webs. 

For  cross-girders,  it  shall  be  the  perpendicular  distance  be- 
tween central  planes  of  trusses. 

For  columns  and  posts,  it  shall  be  tlic  greatest  length  be- 
tween points  of  axis  that  are  rigidly  held  in  the  direction  in 
wliich  the  strength  is  being  considered. 

These  elfeclive  lengths  are  to  be  used  in  calculating  mo- 
meuts,  stresses,  and  working  strengths. 


EFFECTIVE  DEPTHS. 


>%.' 


Effective  deptlis  shall  be  as  follows : 

For  pin-connected  trusses,  the  perpendicular  distance  be- 
tween gravity  lines  of  chords,  which  lines  must  pass  through 
centres  of  pins. 

For  plate-girders  and  open-webbed  riveted  girders,  the  per- 
pendicular distance  between  centrelines  of  gravity  of  upper 
and  lower  flanges;  but  never  to  exceed  the  depth  from  out 
to  out  of  flange  angles. 


STYLES  OF   BUIDGES   FOU   VAIUOUS   SPAN    LENGTHS. 

For  spans  under  fifteen  (15)  feet,  rolled  I  beams. 

Fc"  spans  between  fifteen  (15)  feet  and  eighty-five  (85)  feet, 
plate  girders. 

For  sjmns  between  eighty-five  (85)  feet  and  one  hundred  and 
twenty-five  (125)  feet,  "A"  truss,  pin-connected  spans,  or  riv- 
eted, open-webbed  girders  of  single  cancellation. 

For  spans  between  one  hundred  and  twenty-five  (125)  feet 
and  one  hundred  and  seventy-five  (175)  feet,  riveted,  open- 
webbed  girders  of  single  cancellation,  or  pin-connected  trusses 


14« 


])K  i'()NTini;s. 


tles'gnc'd  willi  speciul  reforcuce  to  cxlieino  rij^idity  in  till 
parts. 

For  spans  cxcei'diiig  one  huiulicd  und  .sevenly-tive  (175) 
feet,  piii-conne(;tt'd  spans. 

The  use  of  pony  truss  bridges  of  any  iiind  is  proliibited, 
excepting  only  halfllirougli,  plute-giider  spans,  in  whicii  ilus 
top  llunges  me  held  rigidly  in  phue  by  bmcliets  riveted  to 
cross-gilders  tliat  are  spaei  d  generally  not  to  exceed  liflcen 
(15)  feet  apart. 

In  general,  double-trick  I)ridges  shall  have  only  two  lrus.se.s, 
iu  order  to  avoid  spreading  the  tracks. 

KOIl.MS   OK    TKi;SHKrt. 

The  forms  of  trusses  to  be  used  are  as  follows  : 

For  pin-connected  spans  up  to  one  hundred  and  twenty - 
five  (12.1)  feel,  the  "A"  truss. 

For  opeu-vvel)bed,  riveted  girders,  tlie  Warren  or  tri.'ingulnr 
girder  with  verticals  dividing  the  panels  of  the  top  chords; 
also  the  Pratt  truss. 

For  deck-spans  having  top  chords  supporting  wooden  ties, 
the  Warren  or  triangular  girder  with  verticals  dividing  the 
panels  of  the  top  chords. 

For  spans  between  oni;  hundred  and  twenty  five  (l2o)  fet!t 
Hud  about  two  luindred  and  fifty  (250)  feet,  Pratt  trusses  with 
top  chords  either  straight  or  polygonal, 

For  spans  exceeding  two  hundred  and  fifty  (250)  feet,  Petit 
trusses. 

It  is  understood  that  these  linnting  lengtiis  are  not  fixed  ab- 
solutely, as  the  best  limits  will  vary  somewhat  with  the  num- 
ber of  tracks  and  weight  of  trains. 


MAIN   MEMBEllS  OK  TKU8S-BUID0K8. 

All  spans  of  every  kind  >hall  have  end  floor-beams,  riveted 
rigidly  to  the  trusses  or  girders,  for  supporting  the  stringers. 
Stringers  are  to  be  riveted  to  the  webs  of  the  cross-girders. 
Iu  general,  all  trusses  shall  have  main  end  posts  inclined. 
All  trusses  shall  be   so  designed   as  to   admit  of   Jicr-urate 


GKNKUA'.    DESCllirTION. 


147 


ciilciilatioii  uf  nil  ulrcsscs,  excopting  only  sucli  iiniiiipurlant 
cases  of  ambiguity  us  that  iiivo'vud  by  using  two  Htiff  diag- 
onals in  a  middle  panel. 

All  liiteral  bracing  and  other  sway-bracing  siiall  be  rigid 
both  above  and  below,  i.e.,  tlie  sections  nuist  be  capable  of 
resisting  compression,  adjualable  rods  for  such  bracing  being 
allowed  only  in  towers  of  druw-spans  and  in  lower  lateral  sys- 
tems of  deck-bridges. 

T)je  stilt  diagonals  of  lower  lateral  systems,  which  shall  be 
of  double  cancellation,  shall  be  riveted  rigidly  to  the  string- 
ers where  they  cross  them,  so  as  to  transfer  in  an  effective 
nuinncv  the  thrust  of  braked  trains  to  the  truss-posts  without 
causing  a  horizontal  bending  on  tlie  cross-giniers. 

All  through-spans  shall  have  stiff  portal  I)racing  at  each 
end,  connected  rigidlv  to  the  inclined  end  iK)Sts.  The  said 
portal  bracing  shall  be  made  as  deep  as  the  specified  dcr  head- 
room will  allow. 

When  the  height  of  the  tru.ssos  is  great  enough  to  permit  it, 
thet^  shall  Imj  used  at  each  panel  point  a  rigid  bracing  frame 
riveted  to  the  top  lateral  strut  and  to  the  posts,  and  carried 
down  to  the  clearance  line.  When  the  truss  depth  is  not 
great  enough  for  this  detail,  corner  brackets  of  proper  size, 
strength,  and  rigidity  are  to  be  riveted  between  the  posts  and 
the  upper  lateral  struts. 

Deck-bridges  shall  liave  stiff,  diagonal  braces  between  oppo- 
site vertical  posts,  which  bracing,  as  a  matter  of  precaution, 
shall  have  suflicient  strength  to  carry  one  half  of  a  panel-truss 
live  load  with  its  impact  allowance  ;  and  the  transverse  bracing 
between  the  vertical  or  inclined  posts  at  each  end  shall  be 
sutticicntly  strong  to  transmit  properly  to  the  masonry  one 
half  of  the  wind-pressure  and  centrifugal  load  (if  there  be 
any)  which  is  carried  by  the  entire  upper  lateral  system  of  the 
span. 

The  lower  lateral  systems  of  deck-bridges  shall  be  made  of 
adjustable  rods  in  alternate  panels,  thus  leaving  every  other 
panel  unbraced,  and  forcing  the  wind-pressure  from  below  up 
the  vertical  bracing  and  to  the  ends  of  the  span  by  the  upper 
lateral  system. 


148 


DK    PONTIBL'S. 


Suspenders  or  hip  verticals  and  two  or  more  piinel  lengths 
of  bottom  chord  nl  each  end  of  each  span  sliall,  preferably,  be 
made  rigid  members,  excepting  that  in  "  A ''  trusses  the  bottom 
chords  and  i  ni'ic;  verticals  are  to  be  of  eye-bars. 

A.l  floor-b(  a  ns  are  to  be  riveted  to  the  truss-posts  in  truss- 
spans,  except. ng  in  the  case  of  Petit  trusses  when  the  suspenc'- 
ers  are  of  eye  bars.  In  these,  tioor  beam  hangers  may  be 
i:sed,  provided  they  be  made  of  plates  or  shapes,  and  that  they 
be  stayed  at  their  upper  ends  against  all  possibility  of  rotation. 


! 


CONTINUOUS   SPANS. 

Except  in  tlie  case  of  swing-bridges  or  cantilevers,  constu-u- 
tive  spans  are  not  to  be  made  continuous  over  the  points  ol 
support. 

TKESTLK   TOWEU8. 

As  a  general  rule,  each  trestle- bent  shall  be  composed  of 
two  columns  battered  from  one  and  a  half  (Ij)  to  two  and  a 
half  (21)  inches  to  the  foot,  the  bents  being  united  in  pairs  to 
form  towers.  Ench  tower  thus  formed  shall  be  thoroughly 
brac-'d  with  rigid  bracing  on  all  four  faces,  and  shall  hnve 
four  horizontal  struts  at  the  base.  In  each  intermediate  liori- 
zoutal  plane  of  division,  formed  by  the  panels  of  the  tower 
bracing,  there  is  to  be  a  pair  of  diagonal  adjustable  rods  to 
bring  the  colunms  into  proper  position  and  to  retain  them 
there. 

The  feet  of  the  columns  must  be  attached  to  anchorages 
capable  of  resisting  twice  the  greatest  possible  uplifting  ;  and 
the  details  of  the  melalwork  connecting  the  anchor-rods  to 
th<;  columns  must  be  such  as  to  make  the  metal  woik  and 
pedestals  act  as  a  single  piece,  so  that,  if  tested  to  destruction 
by  overturning,  the  bent  would  uol  fail  between  the  super- 
structure and  the  substructure. 

While  it  is  desirable  to  have  suflicient  l)ase  to  prevent  any 
tension  from  coming  on  the  anchor-bolts,  it  is  not  advisable 
on  this  account  to  nuike  the  batter  of  the  columns  too  great, 
especially  in  very  high  trestles. 


GKNERAL   DESCllIPTION. 


149 


When  trestlo-bents  become  unduly  wide,  a  vertical  column 
is  to  be  jiliiced  midway  belweeu  the  legs  so  as  lo  divide  up 
the  truiisverse  and  horizontul  sway-biacing. 

Care  must  be  taken  to  jv.ovide  properly  for  expansion  and 
contraction  at  column  feet  both  transversely  and  longitudinally. 

In  elevated  railroads,  the  towers  can  be  placed  at  about 
every  fourth  span  or,  say,  every  one  hundred  and  fifty  feet, 
or  can  be  dispensed  with  altogether,  when  the  conditions  so 
re(iuiie,  by  strengthening  the  columns  properly  to  resist 
traction,  thrust  of  braked  trains,  and  the  longitudinal  compo- 
nent of  diagonal  wind-pressure. 

ADJUSTABLE   MEMBERS. 

It  is  preferable  to  avoid  altoget.icr  the  use  of  adjustable 
members  in  trusses,  as  well  as  in  sway-bracing.  If  the  struc- 
ture must  hv  made  as  cheap  as  possible,  adjustable  counters 
may  be  employed  ;  but  it  ix  advisjvble  to  confine  their  use,  as 
before  stated,  to  diagonals  in  towers  of  swing-spans  and  in 
lower  lateral  systems  of  deck-bridges. 

CAMKEU. 

All  trusses  must  be  provided  witii  such  a  camber  that,  with 
the  heaviest  live  load  on  the  span,  the  total  camber  shall  never 
be  quit(!  taken  out  by  deflection.  With  parallel  chords,  sutti- 
cient  (imb(!r  will  be  obtained  by  making  the  top-chord  sections 
longer  than  the  corresponding  bottom-ehord  sections  by  one 
eighth  (i)  of  an  inch  for  each  ten  (10)  feet  of  length.  One 
half  of  the  camber  after  a  span  is  swung  is  to  be  taken  out  of 
th(!  trac!'  by  dapping  the  lies,  unless  this  would  cut  too  deeply 
into  the  timber. 

Plate  girders  and  shallow,  open-webbed,  riveted  girders 
should  not  be  given  any  camber. 

EXPANSION. 


Ever}'  span  nmst  be  provided  with  some  means  of  longitud- 
inal expansion  and  contraction  due  to  changes  of  temperature 


150 


DE  PONTIBUS. 


over  a  range  of  cue  lumdrcd  mid  fifty  (150)  degrees  Fabreii 
heit. 

Spans  up  to  eighty-five  (85)  feet  in  length,  or  in  certuiii 
cases  up  to  even  one  hundred  (100)  feet,  may  slide  on  pinned 
surfaces;  but  those  of  greater  lengtli  must  move  on  nests  of 
turned  rollers.  Occusionally  a  rocker  end  is  permissible;  but 
this  method  of  expansion  ia  always  to  be  avoided  if  practi- 
cable. 

ANCHOUAGE. 

Every  span  must  be  anchored  at  each  end  to  the  pier  or 
abutment  in  such  a  manner  as  to  prevent  the  slightest  lateral 
motion.  Vut  so  as  not  to  interfere  with  the  longitudinal  motion 
of  the  trusses  or  girders  du(!  to  changes  of  tcmperaiure  or 
loading. 

NA.HE  IT.ATES. 

Tlie   niimes  of   the  designer,   manufacturer,  and  builder  of 
every  bridge  or  trestle,  also  the  dute  of  erection,  must  l)c  :it 
tached  thereto  in  a  prominent  position  and  iu  a  durable  man- 
ner. 

LOADS. 

The  loads  to  be  considered  in  designing  bridges,  trestles, 
and  elevated  railroads  are  the  following;  and  all  parts  of  same 
are  to  be  proportioned  to  sustain  properly  the  greatest  stresses 
produced  thereby  for  all  possible  combinations  of  the  various 
loads. 

A.  Live  Load. 

B.  Lnpact  Allowance  Load. 

C.  Dead  Load. 

I).  Direct  Wind  Load. 

E.  Indirect  Wind  Load,  or  Transferred  Load. 

F.  Traction  Load. 

G.  Centrifugal  Load. 

IL  Elfeets  of  Changes  of  Temperatu/e. 

In  calculating  the  stresses  caused  Ijy  a  uniform  moving 
load,  the  load  shall  be  assumed  to  cover  the  panel  in  advance 
of  the  panel  point  considered;  but  the  half-panel  load  going 


LOADS. 


151 


to  the  forward  pjiiicl  point  will  be  ignored;  or,  in  other  words, 
tlie  uiiiforni  load  will  lie  treated  as  if  eoiiceutrated  at  the 
various  panel  points. 

In  deck-spans  on  .sharj)  curves,  after  the  centre  curve  for 
each  rail  and  Hit;  centre  lines  of  the  longitudinal  girders  are 
laid  out,  th(!  approximate  extra  live  load  on  the  outer  girder 
due  to  tin;  projection  of  the  curve  of  the  rail  beyond  its  centre 
line  near  nnd->|)an  is  to  be  computed  and  added  to  the  regular 
live  load;  but  the  corresponding  excess  of  dead  load  from  the 
flooring,  being  .small,  is  to  be  ignored.  As  the  superelevation 
provides  for  an  ecjual  distribution  of  the  live  load  on  the  rails 
for  tlic  assumed  medium  velocity  of  trains,  there  will  be  au 
excess  of  live  load  on  the  outer  girder  due  to  the  velocity  be- 
ing sometimes  arealer  tlum  this;  but  the  said  excess  is  so 
small  that  it  is  to  be  ignored. 

The  excess  of  live  load  on  the  inner  girder,  due  to  the  ve- 
locity of  train  being  sometimes  less  than  that  assumed  for  de- 
termining the  superelevation,  is  offset  by  the  reduced  load 
due  to  the  projection  of  the  centre  line  of  the  rail  near  mid- 
span  beyond  the  centre  Hue  of  the  girder;  so  it  also  is  to  be 
ignored. 

LIVE   LOADS. 


The  live  load  to  be  used  in  designing  any  railroad  structure 
shall  be  taken  from  the  "Compromise  Standard  System  of 
Live  Loads  for  Railway  Hridges  and  the  Equivalents  for 
Same,"  which  is  given  in  Chapter  XIX  and  in  Plates  I,  II, 
III.  and  IV. 

lu  single-track  bridges  but  one  of  the  seven  classes  of  load- 
ing given  can  be  used  for  any  span;  but  in  bridges  liaving 
more  than  one  track  two  or  even  three  cla-sses  of  loading  can 
be  used  in  tin;  same  span,  if  so  desired  bj'  the  Engineer:  for 
insumce,  (.'lass  W  could  be  adopted  for  stringers,  Class  X  for 
cro.ss-girders,  and  Class  V  for  trusses,  thus  utilizing  the  theory 
of  probabilities. 

The  etiinvidcnt  live  loads  given  on  tlie  diagrams  are  to  be 
used  instead  of  the  actual  wheel  concentrations. 

For  elevated  railroads  the  live  loads  are  generally  to  be  very 


152 


I)K    PONTIBUS. 


much  lighter  than  that  of  Class  Z  of  the  Compromise  Stniid- 
iiril  System;  but  ihe  said  loads  will  Imve  to  be  determined  for 
each  iudividiial  system  of  elevated  railroad,  so  iis  to  provide 
for  the  greatest  train  load  thai  can  ever  come  upon  the  struc- 
ture, but  for  no  more. 

IMPACT  ALLOWANCE   LOAD. 

The  impact  allowance  load  is  to  be  a  percentage  of  the 
equivalent  uniform  live  load,  found  by  the  formula 


P  = 


40000 
L  +  500' 


where  Pis  the  porccntago  jind  /,  the  length  in  feet  of  span  or 
portion  of  span  tluit  is  covcicd  b_v  the  live  load,  when  the 
member  considered  is  subjected  to  its  maximum  stress. 


DKAO    LOAD. 

The  dead  load  is  to  include  ilio  we'ght  of  all  the  metal  and 
wood  in  the  structuie,  t'.xccpting  lliat  of  those  portions  resting 
directly  on  the  abulmonts,  whose  wciuflits  do  not  aflfect  the 
stresses  in  the  trusses;  alsci  uiiy  other  pernianenl  load  that  may 
be  carried  l)y  tin;  strueturc. 

The  following  uint  weights  are  to  be  assimied  in  estimating 
the  dead  load : 

Creosoted  lumber  four  and  one-half  (4i^)  pounds  per  foot 
board  measure. 

Oak  and  other  hard  woods  four  and  a  quarter  (4^)  pounds 
per  foot  board  measure. 

Yellow  pine  three  and  three-quarters  (3})  pounds  per  foot 
board  measure. 

Wliite  pine  and  other  soft  woods  two  and  three-quarters (SJ) 
pounds  per  foot  board  measure. 

Kails  and  their  fastenings,  sixty  (60)  pounds  per  lineal  foot 
per  track. 

Two  thirds  (|)  of  the  dead  load  shall  be  assumed  to  be  con- 
centniled  at  the  panel  points  of  the  lower  chords  in  through- 


LOADS. 


153 


bridges  and  at  tliose  of  the  upper  chords  in  deck-bridges;  and 
one  third  (l)  of  the  dead  load  at  the  panel  points  of  the  upper 
cliords  in  through-bridges  and  at  those  of  the  lower  chords  in 
deck-bridges. 

If  in  any  bridge;  design  the  dead  load  assumed  should  differ 
from  that  computed  from  the  diagram  of  sections  and  the 
detail  drawings  by  an  amount  exceeding  one  (1)  per  cent  of 
the  sum  of  the  c(iuivalent  live  load  and  actual  dead  load,  the 
calculations  of  stresses,  etc.,  are  to  be  made  over  with  a  new 
assumed  dead  load. 


WIND   LOADS. 

For  railroad  bridges  the  wind  loads  per  lineal  foot  of  span 
for  botli  the  loaded  and  the  unloaded  chords  are  to  be  taken 
from  the  curves  given  in  Plate  VII. 

The  wind  loads  for  the  loaded  chords  include  a  pressure  of 
three  hundred  (300)  pounds  per  lineal  foot  on  the  train,  the 
centre  of  wiiich  pressure  is  applied  at  a  height  of  eight  (8)  feet 
al)ove  the  i)ase  of  rail. 

For  determining  the  requisite  anchorage  for  a  loaded  struc- 
ture, the  train  of  empty  cars  shall  be  assumed  lo  weigh  one 
thousand  (1000;  pounds  per  lineal  fool. 

In  trosth;  towers  (lie  colunuis  iind  transverse  bracing  shall 
be  proportioned  to  resi,st  the  i'nilowing  wind-pressures  in 
addition  to  all  other  loads. 

1st.  When  the  structure  is  loaded,  four  hundred  and  fifty 
(450)  pounds  per  lineal  foot  on  stringers  and  cars,  and  two 
hundred  ami  fifty  (250)  pounds  for  each  vertical  foot  of  each 
entire  tower. 

2d.  When  the  structure  is  empty,  three  hundred  and  fifty 
(IMO)  pounds  per  lineal  foot  on  stringers,  assumed  to  be  con- 
centrated one  foot  above  the  centre  of  stringer,  and  three 
hundred  and  tlfly  (350)  pounds  for  each  vertical  foot  of  each 
entire  tower. 

The  wind  loads  for  longitudinal  bracing  are  to  be  taken  aa 
seven  tenths  (0.7)  of  those  for  the  transverse  bracing. 

In  flgiuing  greatest  tension  on  colunuis  and  anchor-bolts, 
computations  are  to  he  made  for  bolh  tiie  loaded  ami  the  un- 


154 


DE   PONTIHUS. 


loaded  slructure,  in  douhh-tiack  trestles  placing  IJjo  tialu  of 
empty  ears  on  tlie  leeward  Iraek. 

All  wind  l<«uls  are  to  be  treated  as  moving  loads 

INDIRECT   WIND   I-OAD   OU  THAN8FKKRED  liOAD. 

For  both  throui^h  and  deek  spans,  even  with  polygonal  top 
chords,  the  transferred  load  is  to  be  assumed  to  produce  a 
tension  in  the  leeward  bottom  elioid  that  is  constant  from  end 
to  end  of  span,  and  a  similar  release  of  tension  on  the  wind- 
ward bottom  chord.  For  trusses  with  parallel  chords  this 
assunii)tioii  is  correct,  provideil  that  all  tlie  windjiressure 
travels  directly  to  ends  of  span  by  the  horizontal  bracing;  wiiile 
for  trusses  with  polygontd  top  chords  the  asstnnption  is  a  com- 
promise, the  travel  of  wind-pressure  being  ambiguous.  The 
transferred  load  at  one  pedestal  is  to  be  found  by  multiplying 
one  half  of  the  total  wind  load  on  the  top  chord  by  the  average 
truss  depth  and  dividing  the  product  by  the  perpendicular 
distance  between  central  planes  of  trusses. 

TRACTION    LOAD. 

The  total  traction  load  on  anj^  portion  of  a  structure  is  to  be 
taken  as  twenty  (20)  per  cent  of  the  greatest  live  load  that  can 
be  placed  on  that  portion  of  .said  structure. 

In  proportioning  the  towers  and  colunuis  of  trestles  and 
elevated  railroads,  llie  towc-rsand  columns  between  consecutive 
expansion  points  are  to  be  assumed  to  receive  no  aid  from 
neighboring  towers  and  columns,  but  must  be  figured  for  the 
greatest  possible  traction  load  between  said  consecutive  expan- 
sion points. 

No  percentage  of  impact  is  to  be  added  to  traction  roads. 

CENTIUFUOAL   LOAD. 

The  centrifugal  load  is  to  be  computed  for  the  greatest 
probable  velocity  of  trains  by  the  formula 


C 


wv' 


INTENSITIES  OF   WORKING-STRESSES. 


155 


wlicrc  iJ  is  the  ceutrifiigul  load  ptr  liueiil  foot,  w  is  the  cquiv- 
iilcMit  live  loud  per  lineal  fool,  v  is  the  velocity  of  train  in  feet 
jwr  second,  and  li  is  the  radius  of  the  curve  in  feet. 

All  portions  of  the  structure  affected  by  the  centrifugal 
lo!i(l  are  to  be  figured  to  carry  properly  the  stresses  induced 
by  the  said  load  in  addition  to  all  other  stresses  to  which  they 
may  be  subjected. 

No  percentage  of  impact  is  to  be  added  to  centrifugal  loads. 

EFFECTS  OF  CHANGER  OF  TEMPERATUUE. 

In  ordinary  structures  changes  of  temperature  will  not  affect 
the  stresses  in  the  menjbers,  provided,  of  course,  that  proper 
l)recuulion  be  taken  to  permit  unrestricted  expansion  and  con- 
traction. But  in  all  arches,  excepting  only  those  hinged  at 
both  ends  and  at  the  crown,  the  stresses  caused  by  the 
assumed  extreme  changes  of  temperature  must  be  computed 
and  duly  considered. 


INTENSITIES  OP  "WOBKING-STRESSES. 

The  following  intensities  of  working-stresses  (i.e.,  pounds 
per  Sipiare  inch  of  cross-section)  are  to  be  xised  for  all  cases, 
except  where  wind  loads  are  ctmibined  with  other  loads,  under 
which  conditions  the  said  intensities  are  to  be  increased  thirty 
(;{())  per  cent.  But  when  high  steel  is  employed  the  metal  is 
to  be  strained  tifteen  (15)  i)er  cent  higher  for  all  ca.ses  than 
herein  specified,  even  after  the  said  thirty  (30)  per  cent  has 
been  added  to  allow  for  wind  stresses. 

Tension   on   eye-bars  in    bottom   chords    and 

main  diagonals,  and  on  lateral  rods 18,(X)0  pounds. 

Tension   on    shapes  in   bottom   chords,  main 

diagonals  and  laterals,  on  eye-bars  in  sus- 

^)ende^s  and  hip  verticals,  and  ou  soft-steel 

adjustable  truss  members 16,000      '* 

Tension  on  net  section  of  jdate-girder  fianges 

(assuming  one  eighth  of  the  area  of  the  web 

to  act  as  a  part  of  each  fiange),  extreme  fibres 


156  1)E   PONTIBUS. 

of  rolled  I  benms,  ami  on  shni)es  in  body  of 

suspenders,  hip  veilicals  and  hanger-plates 

(there  being  50  per  cent  increase  of  net  area 

for  section  througii  eyes) » 14,000  pounds 

Tension     on    adjustable     truss     members    of 

wrought  iron •  •     13,000 

IJending  on  pins 37,000 

Bearing  on  pins  and  rivets  (measured  upon  the 

projection    of  the  sennintrados  upon  a  di 

ametral  plane) 22,000 

Shear  on  pins  and  rivets 12,000 

Shear  on  webs  of  plate  girders 10,000     ' ' 

For  fleld-rivels  the  intensities  for  bearing  and  shear  are  to 
be  reduced  twenty-flve  (35)  per  cent. 

Compression  on  top  chords 18,000  —  70  - ; 

I 
Compression  on  inclined  end  posts. . ,   18,000  —  80  -; 

Compression  on  all  other  struts  with 

I 

fixed  ends 16.000  -  60-; 

r 

Compression  on  all  other  struts  with 

I 
one  or  two  hinged  ends 16,000  -  80  -; 

T 

where  I  is  the  unsui>ported  length  of  the  strut  in  inches  and  r 

is  its  least  radius  of  gyration  in  inches. 

Compression  on  end  stiffeners  of  plate  girders.  14,000  pounds. 

Tension  on  extreme  fibres  of  long  leaf, 
Southern,  yellow-pine  timber  in  bending, 
the  effect  of  impact  being  considered 2,000      " 

BEARINGS   DPON   MASONRY. 


All  bed-plates  must  be  of  such  dimensions  that  the  greatest 
pressures  on  the  masonry,  including  impact,  shall  not  exceed 
those  iriveii  in  the  following  table: 


INTEN81T1KS    OF    W()UKlN(J-STIlK8Si:S. 


157 


Matorial  I'eiiiiissible  Pressure 

*'**®"*'-  per  Square  Inch. 

Am.  Nat.  Cement  Concrete 130  pounds. 

Brickwork  laid  in  Cement 170  " 

Portland  Cement  Concrete 200 

Ordinarily  Good  Sandstone 200  " 

Extra  Good  Sandstone 250  " 

Yellow  Pine  or  Oak  on  Flat 300  " 

Ordinarily  Good  Limestone 300  " 

Extra  Good  Limestone 400  " 

Granitoid 450  " 

Granite 550  " 

KEVEK8IN«-8TUE8SE8. 

In  case  stresses  reverse,  the  areas  required  for  both  tension 
and  compression,  including  impact  in  each  case,  are  to  be 
fl;jrured  separately,  and  three  fourths  (J)  of  Ihe  smaller  urea  is 
to  be  added  to  the  larger  are.i  in  order  lo  obtain  the  total 
sectional  area  of  the  piece.  The  rivets,  however,  are  lo  be 
figured  for  the  sum  of  the  two  stresses,  both  impacts  included. 

The  effect  of  reversion  of  stresses  in  case  of  wind  loads  is  lo 
be  ignored  when  computing  sectional  areas  of  members  and 
the  number  of  rivets  required  ;  but,  of  course,  wherever  rever- 
sion of  stress  occurs,  the  piece  must  be  stiffened  so  ns  to  resist 
compression. 

NET   SECTION. 

The  net  section  of  any  tension  Hange  or  member  shall  be 
determined  by  a  plane  culling  the  member  square  across  at 
any  point.  The  greatest  luunber  of  rivet-holes  whicli  can  be 
cut  by  any  such  plane,  or  whose  centres  come  nearer  than  two 
and  a  half  (2i)  inches  to  saiil  phuie,  are  to  be  deducted  from 
the  gross  section  when  computing  the  net  area. 

BENDING   MOMENTS  ON    PINS. 


In  figuring  the  bending  moments  on  pins,  the  stresses  shall 
be  assumed  as  concentrated  at  centres  of  bearings. 


168 


DE   PON  Tin  US. 


COMBINATIONS    OP    STRESSES. 

Tn  tlie  girders  of  pl.-ilc  tinier  spans  mid  of  (1»'(U,  o|)t'ii-\\('l)l)('(l, 
rivi'tt'd-irirdor  sp;ins,  tin;  only  stresMes  Mint  nct-d  to  ho  cimi- 
sidorcd  are  tliose  ransed  by  the  live,  iiiipaci,  dead,  mid  cen- 
trifugal loads. 

The  trusses  of  tlirough-hridges  will  be  nlfected  by  the  live, 
impart,  dead,  direct  wind,  and  indirect  wind  loads  ;  and  in 
exceptional  cases  ;i'  >  liy  tlie  centrifugal  load.  The  trusses  of 
deck  bridges  will  l)u  all'ected  by  all  of  Ihewj  loads.  In  uo ca.se 
will  tlie  traction  load  allecl  tiie  trusses  of  bridges  to  such  an 
e.vtent  as  to  reciuire  consideration  ;  conseipicntly  the  only 
l^rovisiou  for  traction  load  required  in  through  and  deck 
bridges  is  adequate  rigid  bracing  to  carry  it  from  tlie  track  to 
the  trusses  without  subjecting  any  portion  of  the  structure  to 
an  improper  loading,  as,  for  instance,  the  flanges  of  cross- 
girders  to  horizontal  bending. 

In  hri(lf/iH  of  all  kinds  the  various  loads  herein  specified 
sliall  be  combined  without  any  reduction  ;  but  in  trestles, 
more  especially  very  liigb  ones,  it  will  be  legitimate,  wlien 
combining  the  stres.ses  from  the  various  loadings,  to  reduce 
some  of  them  or  even  to  ignore  some  entirely,  in  order  to 
avoid  proportioning  for  any  highly  improbable  or  impossible 
combination  of  loads.  For  instance,  when  a  trestle  is  situatid 
near  the  middle  of  a  sharp  curve  or  near  the  apex  of  two 
lieavy  rising  grades,  it  would  be  incorrect  to  assume  a  higii 
velocity  of  train.  In  such  cases  as  these  the  element  of  in- 
dividual judgment  in  combining  the  stresses  from  the  various 
loads  and  in  assuming  the  sizes  of  the  latter  cannot  well  be 
eliminated. 

IJENDING    ON    TOP    cnOKDS. 

For  cotnbined  direct  stresses  and  bending  (m  chords,  the 
moment  is  to  be  computed  by  the  compromise  formula 


CO.MIMNATIONS   OF    STUKHSKS. 


159 


wlu'it'  IFistlic  totiil  tnmsvcrso  load  in  pounds  on  llie  pitice, 
including  impact,,  and  I  is  tin;  longtii  r)f  tlic  jiiccc  in  iiiciics. 

Tlio  extreme  (Ibrc-stross  for  the  conibiuiitioii  kIihII  n(»l  ex- 
ceed Hixtcen  tlioiisand  (16,000)  pounds  ;  and  tlie  nionunt  at 
mid  iKincl  is  Id  be  ussunit d  the  same  in  amount  as  that  at  the 
|)anel  points. 

Top  chords  subjected  to  transverse  loading  should  be  made 
as  deep  as  economy  of  metal  will  permit. 


HKNDING   ON    INCI-INKD    END    POKTP. 

In  i»roporlionin!r  inclined  ciul  posts  of  trusses  of  throuf^h- 
bridges  for  a  combiuntion  of  all  tin-  loiid-.  liciein  specified,  fo- 
gctlier  with  the  l)cnding  caused  by  llie  wind-pressure  \vlii<li 
travels  transversely  down  tin;  ])iece  to  the  pier  or  al)utment.  tiie 
extreme  fibre  nuiy  be  strained  liiirty  (JJOi  per  cent  liiglier  llian 
the  intensify  specified  for  the  direct  (lompression,  the  bending 
moment  being  computed  on  tiie  assumption  that  the  inclined 
end  post  is  fixed  al)ove  by  the  portal  bracing  and  at  the  bottom 
1)}' its  connections  to  the  pedcsiul  and  end  flo'ir-beam,  Xhw^ 
nniking  the  lever-arm  of  the  moment  equal  to  one  half  the 
length  of  that  portion  of  the  inclined  end  post  lying  between 
the  centre  of  pedestal-pin  and  the  centre  of  the  lower  portal 
strut  (or,  in  case  of  plate-girder  portals,  the  bottom  of  the  said 
plate  girder). 


BENDINCl    DUK    TO    WEIGHT    OF    MEMUKK. 


If  the  extreme  fibre-stress  resulting  from  the  bending  due  to 
the  weight  only  of  any  member  does  not  exceed  ten  (10)  \>er 
cent  of  the  specified  intensity  of  working-stress,  the  effect  of 
such  bending  may  be  ignored  ;  but,  if  it  does  so  exceed,  its 
effect  niu.st  l)e  combined  witli  tliose  of  the  other  stresses, 
vising,  however,  foi- determining  the  sectional  area,  an  inten- 
sity of  worliing-stress  ten  /lO)  per  cent  greater  than  that 
specified. 


1()0 


l)E   I'ONTIIJUS. 


GENERAL    LIMITS    IN    DESIGN  INO. 

The  following  ^eiuMiil  limits  sliall  be  lulhered  to  in  design- 
ing bridges,  I  resiles,  viaducts,  uud  the  line-work  of  elevated 
railroads  : 

No  nieial  less  than  three-eighths  (f)  of  an  inch  in  thickness 
shall  be  used  except  for  lilling-plates. 

Tlie  least  allowable  thicknesses  of  webs  of  rolled  1  beams 
shall  be  us  follows  : 

24"  I  beams f"  webs. 

ao'     •'     vv     " 

18"       "      i"      " 

15"       "       iV'      " 

13"       "       t"      " 

No  channel  less  than  Icn  (10)  inclus  in  deplli  shall  be  used, 
except  for  lateral  stmts,  in  which  eight  (8)  inch  channels  may 
be  employed. 

No  angles  less  than  '■]"  X  ^s  X  §"  s  lall  lie  used,  except  fi)r 
/acing. 

No  eye- bars  less  than  four  (4)  inches  deep  or  three  (juarters 
(J)  of  an  inch  tlii(  k  sliall  be  employed  ;  and  the;  depths  of  eye 
bars  for  chords  and  main  diagonals  shall  not  be  less  than  one 
fifty-fifth  {j'b)"^  t'"^  length  of  the  horizontal  projection  of  same. 

No  adjustable  rod  sliall  have  less  than  one  square  inch  of 
cross-section. 

The  shortest  span  kiigth  for  trusses  with  polygonal  lop 
chords  shall  be  one  huiulred  and  seventy-five  (175)  feet. 

The  limit  of  span  length  in  which  the  stringers  can  be  riv- 
eted continuously  from  end  to  end  of  span  sliall  be  two  hun- 
dred (200)  feet.  Beyond  this  limit  sliding  bearings  nnist  lie 
used  at  one  or  more  intermediate  panel  points  ;  and  in  no 
span  shall  there  be  a  length  of  continuously  riveted  stringers 
exceeding  two  hundred  (200)  feet. 

For  all  couipression-members  of  trusses  and  for  eohnnns  of 
viaducts  an;,  elevated  railroads  the  greatest  ratio  of  unsup- 
ported length  to  least  radius  of  gyration  shall  be  one  hundred 
(100),  excepting  those  menibers  whose  main  function  is  to 


OKNKRAL    I'UINCll'LKS   IN    UESKJNIN'f;. 


Ifil 


resist  tension.     In  those  the  limit  may  be  miscd  to  one  liuu' 
drod  iind  twenty  (liJO). 

The  concspondinf;  limit  for  till  struts  belonging  to  sway- 
bracing  shall  be  one  hundred  and  forty  (140). 


OENERAIi    PRINCIPLES    IN    DESIONINQ 
ALL    STBUCTUBE8. 

In  designing  all  structural  metal-work  the  following  prin- 
ciples are  invariably  to  be  obsrrved  : 

1.  All  members  must  be  straight  between  patiel-poinis,  as 
curved  struts  or  ties  will  under  no  circumstances  be  allowed. 

2.  The  axes  of  all  members  of  irii.ssiisor  gii(l(rs  and  those 
of  lateral  systems  coming  together  at  any  ape.\  of  a  tru.ss  or 
girder  must  intersect  at  u  point,  wiitnever  such  an  arrange- 
ment is  practicable  ;  otherwi.so  the  greatest  care  UMist  be  em- 
ployed to  ensure  that  all  the  induced  strcs'<es  and  bending 
moments  caused  by  the  eccentricity  be  properly  provided  for. 

3.  Truss  members  and  portions  of  truss  members  nuist 
always  be  arrangeil  in  pairs  symmetrically  about  the  central 
plane  of  the  truss,  except  in  the  case  of  single  members,  the 
axes  of  which  lie  in  said  central  plane  of  truss.  Tiiis  applies 
also  to  the  designing  of  open-webbed,  riveted  girders. 

4.  In  proportioning  main  menibers  of  bridges,  symmetry  of 
section  about  two  principal  i>lanes  at  right  angles  to  each 
other  is  to  be  attained  wherever  practic  able;  but  in  designing 
top  chords  and  inclined  end  posts  this  rule  cannot  be  fol- 
lowed. 

5.  In  both  tension  and  compression  members,  the  centre 
line  of  applied  stress  must  iiivarialiiy  coincide  with  the  axial 
right  line  passing  through  the  ceu'res  of  gravity  of  all  cross- 
sections  of  the  member  taken  at  right  angles  thereto. 

C.  The  principle  of  symmetry  in  designing  must  be  ivinied 
even  into  the  riveting;  and  groups  of  rivets  must  be  made  to 
balance  about  centre  lines  and  ceiilra'  planes  to  as  great  an 
extent  as  is  practicable. 

7.  In  all  structural  metal-v/ork,  excepting  only  the  ma- 
ph'mer^   for  operating   jyovaliie   bridges,   uo  torsiojj  on   any 


1G2 


DR    PONTIBIS. 


member  shiill  be  jjermittcd,  it  it  can  possibly  be  avoided ; 
otherwise,  the  gR'»tt\st  care  must  be  taken  to  provide  ample 
streuglh  and  riL'^'ily  for  every  portion  of  the  structure  af- 
fected by  Hiicb  torsion. 

8.  lu  designing  all  pin-connected  work  ample  clearance  for 
packing  must  be  provided,  and  ample  room  must  be  left  for 
assembling  members  in  confined  spaces. 

9.  In  bri(l>;i's,  trestles,  and  elevated  railroads  the  thrust 
from  braked  trains  and  the  traction  must  be  carried  from  the 
stringers  or  longitudinal  gii<lers  to  the  posts  or  columns  with- 
out pr^jducing  any  horizontal  bending  moment  on  the  cross- 
girders. 

10.  In  treslUsand  elevated  railroads,  the  columns  mt'.st  be 
carried  up  to  the  tops  of  the  cross-girders  or  longitudinal 
girders,  and  niust  be  effec'ively  »iveted  theielo.  In  no  case 
will  it  be  permitted  to  cut  oil  the  columns  and  rest  the  cross- 
girders  or  l()ngitudin:d  ginlcrs  on  top  of  same. 

11.  Every  column  that  acts  as  a  beam  also  must  have  solid 
webs  at  right  angles  to  each  other,  as  no  reliance  shall  be 
placed  on  lacing  tc  carry  a  transverse;  load  down  the  column. 

12.  In  trestles  and  elevated  railn)ads,  every  column  must 
be  anchored  so  tirmly  to  its  pedestal  that  failure  by  overttirn- 
ing  or  rupture  could  not  occur  in  the  neighborhood  of  the 
foot  if  the  bent  were  tested  to  destruction. 

13.  The  amount  of  (leld-riveting  must  be  reduced  to  a 
minimiun,  without,  however,  diiniiiisliing  the  number  of 
rivets  recjuisite  for  stiength  and  rigidity.  Whenever  it  is 
practicable,  all  designs  are  to  be  made  so  that  the  ticM-rivels 
can  be  driven  rfadily. 

14.  Rivets  ;ire  not  to  be  used  in  direct  tension. 

15.  For  members  of  any  importan<e,  more  than  two  rivets 
are  to  be  used  for  each  connection. 

16  In  designing  short  membeis  of  open- webbed,  rivete<l 
work,  it  is  belter  to  iiu^reasc  the  sectioni'l  area  of  the  ]ue(  e 
from  ten  (10)  to  twenty-five  ('25)  per  cent  beyoiul  the  Ihcorefi- 
cal  recpnrement  than  to  try  to  develop  tin;  strengMi  by  using 
supplementary  nngl(;s  at  lluf  ends  to  (onncct  to  the  ]ihiic>. 

17.  Slur  SI  ruts  formed  of  two  angles  with  occasiomd  nLojJ. 


OKNEIIAL    IMIINCU'LKS    IN     DIvSKiN  I  N'(i. 


I(i3 


pieces  of  ans^le  or  plafe  for  .stayinj^  same  a'*e  not  to  be  used, 
for  Ix'ttcr  results  iire  oliluiued  by  phiciiig  tiie  augles  in  the 
form  of  a  T. 

IH.  In  all  mail)  members  having  an  excess  of  section  above 
tlial  called  for  liy  the  ureale.s(.  combination  of  stresses,  the 
entire  detailing  is  to  be  proportioned  to  correNpond  with  llie 
utmost  working  capacity  of  liie  member,  ami  not  merely  for 
the  greatest  total  stress  to  which  it  may  be  subjected.  In  this 
connection,  though,  the  reduced  capacity  of  single  angles 
(•oune(;ted  by  one  leg  only  must  not  be  forgotten. 

10.  Di'signs  mu  t  invariably  be  made  j-<>tlial  all  n)etal-work 
after  erection  shall  be  accessible  lo  the  paint-brusli,  (ixcepting, 
of  course,  those  surfaces  which  are  in  contact  svith  eacli  other 
or  with  th(!  masonry.  This  reiiinrement  'ules  out  all  clo.sed 
coiunuis  of  every  type  and  description. 

20.  In  general,  details  must  always  be  proportioned  to  resist 
every  direct  and  indirect  stress  that  may  ever  come  upon  them 
under  any  probable  circumstances,  without  siibjecting  any 
portion  of  their  material  to  a  stress  greater  than  the  legitimate 
(^orresjionding  woi king-stress. 

~M.  In  all  designs  simjjlicity  in  l)otli  main  members  and  de- 
tail-; is  to  be  considered  of  the  greatest  imitortaiu'c. 

'2:1.  In  all  structures  rigidity  is  to  lie  considered  quite  as  im- 
j)ortant  an  clement  as  nnre  strength. 

2;V  Structures  on  skews  are  to  be  avoided  whenever  it  is 
practicable  to  do  so. 

24  The  use  of  more  than  a  single  system  of  cancellation  in 
bridges  shall  be  eontined  entirely  to  lateral  systems  and  sway- 
bracing,  except  that  at  mid-panels  of  trusses  two  rigid  diag- 
<mal.s  conneced  at  theii  intersection  may  for  ajipearance  be 
emi)loyed,  provided  that  either  diagonal  have  sulHcient 
stnngtli  to  carry  the  entire  shear  in  either  tension  or  com- 
pression, and  that  theatljacent  vertical  posts  be  figured  accord- 
ingly. 

25.  Tlie  use  of  redtnidant  meinliers  in  structures  shall  not 
be  allow(;d.  excej'ting  oidy  in  the  case  just  menlioued  of  rigid 
mid  panel  diagonals. 

21)    In  all  designing  true  econnniy  iuia--i  be  sjcivon  the  utmost 


164 


I)E    I'ONTIBUS. 


consideration,  and  no  useless  material  must  beemploj-ed,  cvi'iy 
pound  of  metal  in  tlie  structure  liaving  a  legitimate  function; 
but  economy  of  malerial  must  not  be  (juoted  as  an  excuse  for 
using  inferior  details  or  scamping  tlie  work  in  respect  to 
strengtli,  rigidity,  or  appearance. 

27.  In  nil  structural  work  I  lie  subject  of  restlietics  must  be 
dul}' considered,  and  ail  designs  are  Ui  be  made  in  i)aruu)ny 
witb  tiie  jninciples  thereof,  to  as  great  an  extent  as  the  money 
available  for  the  work  will  permit  or  as  the  euvironment  of 
the  structiue  calls  for. 

RIVETING. 

The  rivets  used  shall  generally  be  seven  eighths  (I)  inch  in 
diameter,  snuvller  ones  being  employed  forsmall  cliannel  flanges 
and  legs  of  angle-irons  less  than  three  and  a  half  (Si^)  inches 
wide.  In  very  heavy  work  IIk;  rivet  dianu'ter  sliould  be  in- 
creased to  fifteen  sixieenlhs  (\l)  inch,  and  in  certain  extreme 
cases  to  one  inch. 

The  least  diameters  for  r'vets  in  flanges  of  channels  are  iis 
follows,  ami  the  greatest  diameters  nuist  not  exceed  the  sanu- 
by  more  than  one  sixteenth  (f^y)  of  an  inch  ; 


Depth  of  Channel 6" 

Diameter  of  Rivet  . . .  5/8" 


5/8" 


8" 
3/4" 


9" 

3/4" 


10" 

■A/i" 


IS" 
3/4" 


15" 

r/s" 


The  i>ileh  of  rivets  in  all  classes  of  work  in  the  direction  of 
the  stress  shall  never  exceed  six  (G)  inches,  or  sixt<  eii  (IG) 
times  the  thickness  of  the  thinnest  outside  plate,  nor  be  less 
than  three  (8l  dianu'ters of  the  rivet.  At  the  ciidsof  compics 
sioii  inenilicrs  ii  shall  not  exerted  four  (1)  times  the  dianu'ter 
of  the  riveLs,  for  a  length  ecjual  to  twice  the  width  of  tin 
meml)er. 

When  two  cjr  more  tldcknesses  or  plate  luc  riveted  togellu f 
in  compiessioii-memlxMs,  the  outer  row  of  livels  shall  not 
be  more  than  four  (4)  diameteis  from  the  side  edge  of  the 
plate. 

Jfo  rivet-hole  centre  shall   be  lejw  ihan   ore  and  a  Imlf  (l^J 


•II 


RIVET  I  NO. 


165 


diameters  from  tlie  edge  of  u  plate,  and,  wlicnevcr  practieable, 
this  distance  is  to  be  increased  to  two  (2)  diameters. 

The  rivets  wlien  driven  must  completely  till  the  holes. 

The  rivet-lieads  must  in  generfd  be  round  ;  ancl  lliey  must 
be  of  uniform  size  for  the  same-sized  livets  lliroriuhoul  the 
work.  They  must  b"  neatly  made  and  (toncentric  with  the 
livct-holes,  and  nuist  thoroughly  pincli  the  connected  p'eces 
together. 

Itivets  with  flat  heads  shall  be  preferred  to  countersunk 
rivets;  the  iicight  or  tiiickness  of  the  Hat  head  shall  be  three 
eighths  (|)  of  an  inch 

Rivets  shall  not  1  -ouutersunk  iu  plates  less  tlan  seven 
sixteenths  (j'^)  of  an  inch  in  thickness. 

Flanges  of  sfingers  and  girders  carrying  the  vertical  load 
from  the  tiesshall  liave their  rivets  spaced  uniform!}'  from  end 
to  end.  and  at  the  miinnuim  distance  emiilo3'ed. 

Whenever  possible,  all  rivets  shall  l)e  machine-driven,  and 
the  machines  must  ])e  capable  of  retaining  tlie  applied  pressure 
until  after  the  upsetting  is  completed. 

Field-riveting  must  be  done  with  a  button  sett :  the  heads 
of  the  rivets  must  be  liemispherical,  and  no  rough  edges  must 
be  left. 

All  rivets  in  splic-e  or  tension  joints  are  to  be  arranged 
synunelrically  so  that  each  half  of  any  tension-member  or 
splice-plate  shall  have  the  same  uncut  area  on  each  side  of  its 
centre  line. 

No  rivet,  excepting  those  in  shoe-plates  and  roller  or  bed 
l)lates,  is  to  have  a  less  diameter  than  the  thickness  of  the 
thickest  plate  through  which  it  passes. 

The  effective  dianuiter  of  any  rivet  shall  be  assumed  the 
same  as  its  diiuneter  before  driving;  but,  in  making  deduc- 
tions for  rivet-holes  in  tension-members,  the  diameter  of  the 
boles  shall  be  assumed  on(>  eighth  (J)  of  an  iiudi  larger  than 
that  of  the  rivet.  In  the  elfective  are.-i  of  riveted  members, 
jtin,  boll,  and  rivet  boles  shall  be  count'.'d  out  for  tension, 
und  boh  and  j)iu  lioles  shall  be  counted  out  for  compression. 


166 


DE    PONTIHUS. 


DETAILS  OF  DESIGN  POK  ROLLED  I  BEAM  SPANS. 

Rolled  I  beiims  used  as  lougitudiriul  girders  shall  have 
preferably  a  depth  not  less  than  one  twelfth  (,'3)  of  the  span. 
They  shall  be  proportioned  by  their  moments  of  inertia. 

I  beam  spans  may  have  eitiier  one  or  t^vo  beams  per  rail. 
In  the  former  case  the  spacing  should  be  si.x  (G)  feet  six  (0) 
inches,  and  in  the  latter  case  two  (2)  feet  six  (6)  iiicl»es  between 
contiguous  girders.  Willi  two  lines  of  stringers  per  track, 
there  will  be  recpiired  a  bracing-frame  at  each  end  of  span  and 
di.'igonal  bracing  between  the  top  llanges,  unless  the  span  be 
less  than  ten  (10)  feet  in  length,  in  which  case  the  diagonals 
may  be  omitted. 

With  four  lines  of  stringers  per  track,  no  diagonal  bracing 
will  be  required,  but  thna'  (:>)  bracing-frames  at  I'iwh  end  will 
be  used,  with  three  (3)  more  at  mid-span  when  the  span 
length  exceeds  ten  (10)  feet. 

Each  I  beam  is  to  have  at  each  end  a  pair  of  stiffening 
anules,  c^ne  of  which  will  form  a  portion  of  the  end  bracing- 
frame.  These  are  to  fit  tightly  at  both  top  and  bottom  against 
the  llanges. 

Under  each  end  of  each  I  beam  there  is  to  be  riveted  a  bear- 
ing plate  of  proper  area  and  thickness  to  jHstribute  the  load 
uniforndy  over  the  masonry,  said  plate  being  bolted  effectively 
to  the  latter  with  due  provision  for  expansion  and  contraction. 


DETAILS  OP  DESIGN  FOR  PLATE  GIRDER  SPANS, 

Plate  girders  shall   have   preferably  a  depth  not  less  than 
one  tenth  ( j*jj)  of  the  span. 

All  plale  girder-;,  whenever  it  is  practicable,  shal'  <)udt 
witlKjut  si)lices  in  the  web  ,  and,  whcni  such  l)eco  -i:  neces- 
sary, the  smallest  possible  lunuber  of  .siuie  shall  be  t.dopted. 
Tlie  splice-pl.ites  and  rivets  for  the  splices  shall  be  such  r 
to  develop  in  every  respect  the  full  strength  of  the  net 
section  of  the  web,  the  main  splice-piates  extending  from 
Mange  to  flange  and  having  at  le.asl  two  (2)  rows  of  rivets  on 
each  side  of  the  joint.  In  addition  to  these,  each  tlange  shall 
be  spliced  by  two  cover-plates  on   lop  of  the  vertical  legs  of 


DKTAILS    Ob'    DESIGN    FOR    I'LATlXil  KUEK    SPANS.    i(n 


tlio  lliUige  aiii^lt'S.  Tlicse  iiiiist  be  long  (Miougli  to  develop  by 
tbo  coiinecling  rivets  nl  least  Iweuty-tive  {■i'))  per  cent  more 
than  Ibe  fill,  slreni;tli  <jf  tlicir  net  seclion. 

Splices  in  tlange-plales  and  angles  must  always  be  avoided 
wlien  sulHcicnily  long  plates  and  angles  are  procurable,  wliich 
will  always  he  tlie  case,  unless  llu;  span  l)e  al)tiornially  long. 
Where  liange-s|)lices  are  unavoiilable,  I  hey  must  be  so  lociiled 
that  uo  two  pieces  of  either  the  flange  or  tiie  web  shall  be 
spliced  within  two  (2)  feel  of  each  other,  and  so  that  no 
llange  splice  shall  occur  at  any  point  where  there  is  not  an 
excess  of  sectional  urea  alH)ve  the  theoretical  iequirenieiU>. 
Every  non-continuous  Hange-piece  shall  be  fully  spliced  so  thai 
tlie  splicing  plates  and  rivets  shall  have  a  calculated  streng  ii 
at  least  twenty  live  ('J:"))  pei'  cent  greater  than  that  of  tlie  section 
spliced.  Field-splicing  of  plate  girders  will  never  h^'  allowed 
for  fixed  spans,  except  in  structures  lor  foieign  countries. 

At  least  one  half  of  every  llange  section  must  consist  of 
angles,  or  else  the  heaviest  sections  of  the  latter  iuu.st  be  u.^ed  ; 
and  the  innnbcr  of  cover-plates  must  be  made  as  sin:ill  as 
practical)le,  in  no  c;ise  exceeding  thiee  (3)  per  flange.  The 
lengths  of  ilii'sc  covci  plates  must  be  such  as  to  make  them 
l)i'ojecl  at  each  end  not  less  than  nine  (11)  inches  lieyond  the 
point  dctcrmiiu'd  by  the  calculations  for  the  reiiuisite  resistance 
to  bemling. 

Where  two  or  three  cover-plates  per  flange  are  used,  they 
shall  be  of  <  ({u.-d  thickness,  or  shall  decrease  in  thickness  out- 
ward from  the  angles.  The  cover  plates  shall  not  extend 
more  than  four  (4)  nchcs  or  e'ght  (8)  times  the  thickne-s  of 
the  outer  pl;Uc  beyond  the  ou'er  line  of  rivets.  Willi  cover- 
I>lales  more  than  fourteen  (II)  inches  wide,  four  4)  lines  of 
rive!s  shall  be  used. 

'i'lie  compression. tlaiiges  of  plate  girders  shall  be  made  of 
the  ^anie  gross  section  as  the  tension- flanges;  and  they  shall  be 
.so  siilfened  laterally  that  the  unsupported  length  shall  never 
exceed  twelve  d'J)  limes  llie  width  of  flange. 

In  deck  spans  there  arc  to  l)e  bracing  fraie-e^s  at  t!ie  ends 
nm]  at  int(M-medi;ite  points  not  more  than  fifteen  ( l.^»  feel  apart; 
.and  iliere  is  |o  ho  an  eireeiive  system   of  diagonal   ht-cing  of 


1G8 


DE   PONTIJU.'S. 


iitiglt'S  between  the  toi)  fliiiigos  of  the  contiguous  girders  for 
euch  truck. 

Ill  half-through  npiins  the  girders  are  to  be  divided  up  into 
l)auels  geneially  not  exceeding  fifteen  (lij)  feet  in  leiigtii.  If 
u  steel  floor  system  be  \ised,  tliere  lire  to  be  brucliets  of  web- 
pliites  !iud  angles  at  the  ends  of  the  cross-girders  extending  to 
the  top  liaiiges  of  the  longitudinal  girders,  so  as  to  stay  tlie 
laiter  eft'eclively  ;  while,  if  a  wooden  floor  system  of  ties  rest 
iug  on  shelves  or  on  the  bottom  flanges  L  used,  there  are  to 
be  steel  cross  frames  with  solid  webs,  of  the  greatest  de|)lli 
obtainable,  with  similar  brackets  at  their  ends  for  the  same 
pui'i'ose.  Ilalf-lhroiigh  plate-girder  spans  are  to  have  a  rigid, 
double  intersection,  lower  lateral  system  of  angles  riveted  to- 
gether by  ])lates  and  angles  at  their  intersections  and  to  the 
botlom  flanges  of  ilic  steel  stringers,  if  the  hitter  be  employed. 

Webslillciicrs  shall  be  placed  at  the  ends  of  plate-girder 
spans,  also  iil  all  points  of  concentrated  loading  and  at  inter- 
mediate i>oints  at  di.^tances  not  exceeding  either  the  depth  of 
the  girder  or  Ave  (5)  fiet,  except  in  the  case  of  shallow  girders 
where  the  shear,  including  impact,  does  not  exceed  Ave  thou- 
sand (5000)  pounds  per  square  inch  of  web  section.  Under 
such  circumstances  the  spacing  of  inteiinediate  sliireners  may 
be  made  as  great  as  three  (3)  feet  six  ((5)  inches. 

All  sliireners  must  bear  lightly  at  top  and  bottom  against 
the  flange  angles.  Under  end  stilTcners  tlitre  must  be  flUfra 
flush  with  the  flange  angles,  but  intermediate  slill'eiiers  shall, 
preferidily,  be  crimped. 

End  stilTeiiing  angles  shall  in  no  case  be  less  than  3i"  X  3J  ' 
X  f  ,  net,  and  must  have  siiflicitnt  area  to  curry  the  entire  enii 
shear,  including  impact,  willi  ihe  specified  intensity  of  woik- 
iiig-slrcss,  no  relianci'  being  placed  on  the  fllhT'^. 

The  sections  of  intermediate  si liTening  angles  shall  not  be 
less  than  those  given  in  the  following  tabC. 


I^eiipth  of  Wirder.  Dimensions  of  Angles. 

Up  to  50' n\  X  :H"  X  §" 

From  50'  to  70' 4      X  H     X  | 


From  7t»  to  90' 


X3i 


XI 


avu 

as 

cas 

vati 

wit 

nici 


OPKN-Wi;i?BUI),  KIVKTHl)   (il  i{im:k-sfaxs.        IGD 

In  proportioiiiugllie  flanges  of  plate  girders,  one  eiglitb  (J)of 
tlie  gross  urea  of  the  web  is  to  be  jisstiiiied  !is  coneeiitnited  at 
till!  centre  of  gravity  of  eacii  flange;  or,  in  otlicr  words,  after 
iiaving  found  tlie  net  sectional  area  reiiuired  for  tbe  tension- 
flange  by  ignoring  the  resistance  of  tlic  web  to  Ijcndiug,  tiiere 
is  to  be  subtracted  llierefrom  one  eigbth  (J  of  the  gross  area 
of  the  web- plate. 

At  the  ends  of  all  plate  girders  tliere  must  l)e  suflicient  rivets 
in  eadi  flange  to  transfer  properly  thereto  from  tlie  wei)  the 
total  end-shear  in  a  distance  eijual  to  the  ellective  deptli  of  the 
girder. 

At  the  euds  (,f  cover-plales  the  spacing  of  tiie  rivets  which 
attach  the  covers,  for  a  length  eijual  to  at  least  twice  the  width 
thereof,  shall  be  male  the  minimum  used  in  the  tianges. 

Under  each  end  of  each  plate  girder  there  is  to  be  riveted  a 
bearing  plate  of  proper  area  and  tiiickiiess  and  thf)rongiily 
stiffened  so  as  to  distribute;  the  load  luiiformly  over  the  ma- 
sonry, said  plate  being  bolted  effectively  to  the  latter  with  due 
provisiou  for  expansion  and  contraction. 


DETAILS  OF  DESIGN  FOB  OPEN- WEBBED,  KIVETED 

GIRDER-SPANS. 

All  open-webbed,  riveted  girders  for  both  deck  and  half 
through  bridges  shall  be  riveted  up  completely  in  tin.'  siiop, 
as  flelil-rivetiug  will  be  allowed  only  for  the  lateral  bracing, 
except  in  structures  for  foreign  countries. 

In  open-webl)ed,  through,  liveted  ginUrs,  however,  lluMon- 
nection  of  main  memheis  will  have  to  be  by  ticld-rivets.  In 
such  cases  all  of  the  trus.s-meMil>ers  will  have  to  be  assembled 
in  tiie  shop,  after  which  the  rivet-holes  for  the  cwuieclions 
shall  be  reamed  so  as  to  ensure  perfect  filtintf  in  Iho  tiehl. 

The  u:?e  of  shallow  opcii-weltbed,  liveted  girders  shall  be 
avoided  whenever  possible,  for  the  reason  that  they  aie  tpute 
as  expensive  and  never  as  satisfactory  as  plate  girders.  In 
case,  though,  of  their  being  reipiired,  as  for  instance  in  ele- 
vated railroads  i>ocnpyiiig  city  streets,  they  are  to  be  jyrovideil 
with  short,  substantial  web-plilesat  the  ends  and  al  all  inter- 
mediate points  where  connections  are  made  to  other  girders. 


i:o 


I)K    PONTIHUS. 


lu  i>iop()rtioiiiiig  the  wcb-mcnibers  of  such  girders,  the 
Hpccitied  intensilii's  of  working  stresses  arc  to  be  reduced  fioiii 
ten  (10)  per  cent  for  (»"  X  3,j'  angles  to  twenty-tive  (25)  per 
cent  for  eciuiil-legged  angles,  with  proportionnle  amounts  for 
angles  of  intermediate  inequality  of  legs,  so  as  to  compensate 
for  the  secondary  stresses  due  to  the  eccentric  grip  of  tlie  rivets. 
In  no  case  will  it  be  jierinissible  to  use  lljits  instead  of  angles 
for  web-menibers,  but  tci'S  may  be  employed,  |)rovided  their 
heads  be  wide  enough  to  permit  of  satisfactory  riveted  con- 
nections. 

A.t  all  intersections  of  web-members  with  chords,  connect- 
ing phites  are  to  be  used;  for  it  is  not  perniiss'lile  to  attach 
web  angles  directly  to  chord  angles  willujut  using  an  inter- 
mediary ])late. 

The  exact  intersection  at  a  point  of  all  gravity  lines  of  girder- 
members  assembling  at  any  apex  must  l)e  adhered  to  in  tiie 
designing  of  open-webbed,  riveted  girders. 

In  designing  all  riveted  connections,  the  greatest  care  is 
to  be  taken  to  make  connecting  plates  and  groups  of  rivets 
l)alance  about  centrelines  of  stress,  especially  where  pa><siiig 
from  riveted  work  to  pin-connected,  as  in  the  case  of  a  rivet,ed 
span  witii  hinged  ends  at  pedestals. 

In  all  other  particular^',  llu^  designing  of  open-webbed, 
riveted  work  is  to  comply,  wiu'rcvcr  praclicable  and  proper, 
with  the  specifications  for  lyiale-girdcr  and  pin-connected 
spans. 

DETAILS  OP  DESIGNS  POH  PIN-CONNEOTED  SPANS. 


The  sections  of  the  top  chords  and  those  of  the  inclined  end 
posts  of  through-spans  shall  consist  generally  of  two  built 
channels  and  a  (;overi)late,  each  channel  iieing  formed  of  a 
web  and  two  angles,  the  upper  one  small  .-I'ld  the  lower  one 
much  larger,  so  as  to  l)ring  tiie  centre  of  giavlly  of  tiie  entire 
box  section  of  the  nuMuljcr  as  close  as  possible  to  the  mid- 
plane  of  the  \veh-|)lates.  In  no  ciise  will  more  than  one  cover- 
plate  be  allowed,  and  this  is  to  be  made  as  thin  as  is  proper. 
It  is  permissible  to  substitute  rolled  channels  for  the  built 


r)p:TAIL.S  OF    DKSIO.V  FOIl  I'lN-CONNRi^TEl)    SPAKS.    171 


ones ;  l)iit,  wlicti  tliis  is  done  it  is  often  iitlvisjible  to  rivet  a 
thick  Miinow  plate  to  the  under  side  of  eiieli  <'huiinel,  in  order 
to  fiicilitale  the  piickinj^  and  detailing  of  \vel)-inenil)ers  by 
keeping  tlie  centre  line  of  stress  coincident  with  the  gravity 
axis  of  the  piece. 

Main  vertical  posts  shall,  generally,  be  composed  of  two 
laced  channels,  ])referabiy  r:)lled  ones,  although  built  ones 
can  be  used  where  large  sections  are  required. 

Hi'condaiy  vertical  [losts  may  be  built  of  two  ndled  channels 
laced,  or  of  four  angles  in  the  form  of  an  I  with  a  single  lint; 
of  lacing.  These  secondary  vortical  posts  should,  preCprably, 
bo  rivete<l  to  the  top  chord  instead  of  being  piu-counected 
like  the  main  vertical  posts. 

The  channels  of  vertical  posts  may  have  their  tlanges 
turned  either  inward  or  outward  as  desired,  or  so  as  to  best 
suit  the  general  detailing  of  the  truss. 

Stiir  bottom  chords  and  in(dined  web-struts  may  be  made 
of  either  two  channels  with  two  lines  of  lacing  or  of  four 
angles  with  one  line  of  lacing. 

Upper  lateral  struts,  overhead  transverse  struts,  and  web- 
stillening  struts  shall,  preferably,  be  made  of  four  angles  with 
one  line  of  lacing.  In  case,  however,  lh(>  said  angles  be  spaced 
very  far  apart,  jis  in  later  il  struts  connecliiig  deep  top  chords, 
they  are  tf)  be  placed  on  the  corners  of  a  rectangle,  wi.h  their 
legs  turned  inward,  and  laced  on  all  four  faces  of  the  box 
.strut  thus  formed. 

Eye-bars  are  to  be  used  for  !ill  bottom  chords  and  main 
diagonals  that  do  not  require  to  be  stiffened. 

Counters,  when  employed,  can  be  of  either  roiuids,  squares, 
or  flats.  These  and  all  other  jidjustable  members  are  to  h;ive 
their  ends  etdarged  for  the  screw-threads  (unless  soft-sleei, 
cold-presse(i  threads  be  used)  so  that  the  diameter  at  the 
bottom  of  the  thread  shall  be  one  eighth  (^)  of  an  inch  greater 
than  that  of  the  body  of  a  round  rod  of  area  equal  to  that  of 
the  atljustable  piece. 

In  short  spans,  two  angles  riveted  back  to  back,  or  even  a 
single  large  angle,  may  be  used  for  lower  Lateral  diagonals  ; 
but  for  long  sp;ins  the  tliagou'ils  nre  to  be  niadc;  of  four  angles 


U'      I 


ir. 


UK    F'ONTIIirS. 


iu  the  form  of  an  I  with  ii  single  line  of  liicing.  When  two 
angles  are  U'^ed,  u  single  i)Iate  must  not  lie  depended  on  to 
form  the  splice  at  the  intvisedion  of  the  diagonals,  but  two 
angles,  each  not  less  tlnn  two  (2j  feet  long,  are  to  be  placed 
benciith  or  on  top  of  the  spliced  angles,  so  as  to  form  ii  full 
splice*  ill  respect  to  rigidit}'  as  w(;ll  as  strength. 

Diagonals  for  upper  lateral  systems  and  vertical  sway- 
bniciiig  shall,  preferably,  be  built  of  four  angles  in  the  form 
of  an  1  with  a  single  line  of  lacing;  hut,  for  structures  where 
this  section  would  involve  an  extravagant  use  of  metal,  two  of 
the  angles,  one  at  top  and  one  at  bottom,  may  be  omitted, 
thus  nuiking  each  strut  consist  of  two  angles  laced,  provided, 
of  course,  thiit  where  the  struts  cross  they  shall  be  rigidly 
connected  by  two  plates  of  ample  size.  This  luibahinced 
section  for  such  diagonals  is  to  be  avoided  Avhenever  it  can  be 
done  without  undue  use  of  metal.  In  no  case,  though,  will 
it  be  permissible  to  use  angles  in  tension  that  are  not  capable 
of  resisting  properly  the  possible  compressive  stresses,  witii 
d\ie  regard  for  the  specified  limit  of  ratio  of  tuisupporled 
length  to  least  radius  of  gyration. 

In  designing  transverse  lateral  and  overhea<l  slrvits  and 
their  connections  it  must  be  remembered  that  their  nuiin 
function  is  to  hold  ligidly  the  chord"  or  posts  to  i)lace  and 
line,  and  not  merely  to  resist  as  columns  th<!  greatest  cal- 
culated direct  stresses  to  which  they  may  be  sunjected.  For 
this  reason  such  struts  should  have  ample  section  for  rigidity, 
and  the  connecting  plates  at  their  ends  shoidd  grip  both  con- 
nected members  effectively. 

Stringers  for  truss-bridges  shall  invariably  be  built  of  plates 
and  angles,  and  no  cover-plates  will  be  allowed  for  the  ttanges. 
Their  depths  shall  be  made  not  less  than  the  most  economic 
ones  in  respect  to  weight  of  metal  lecpiired,  provided  that  the 
bridge  clearance  will  permit,  and  never  less  than  (  le  twelfth 
(^'5)  of  the  span.  No  splices  will  be  allowed  in  tneir  flanges 
nor  any  in  their  webs,  provided  that  sulliciently  long  web- 
plates  are  i)rocurable.  The  compression-flanges  shall  be  made 
of  the  sjune  gross  section  as  the  tension-flanges  ;  audth.-y  shall 


IV. 


DESKJNING   OF    STUINGKUS. 


173 


be  so  slifTciM'il  tlmt  tlie  unsiipportL'd  hjiigtli  Hliall  nuver  exceed 
twelve  (1^')  times  the  widlli  of  tiaiige. 

iii<i;i(l  (tiiigoiiiil  hriiciiig  of  iingleH  is  iiivaiiiihly  to  he  used 
between  I  lie  top  llniiges  of  stiiiigers,  imd  rigid  bniciiig-franics 
lire  to  he  employed  near  all  ex|»!iii.si(>ii  points.  If  the  panel 
length  exeeiul  thirty  (30)  feel,  there  shall  be  a  bra(;ing-fiame 
at  mid-length  l)etweeii  the  contigJious  stringers  of  each  Iraek; 
but  for  all  shorter  jmnels  the  rigid  lower  lateral  diagonals 
which  are  riveted  to  the  bottom  flanges  will  stiffen  the  latter 
sullicienlly. 

In  respect  to  stiffening  anglts  for  stringers,  the  rules 
governing  those  for  plate-girder  spans  are  to  be  followed;  but 
the  end  sliffeners  are  to  be  faced  or  otherwise  treated  so  as  to 
make  the  stiingers  of  e.xael  leigth  throughout,  and  so  as  to 
effect  a  uniform  bearing  of  the  end  stiffeners  against  the  webs 
of  the  cross-girders. 

In  respect  to  pro|)ortioning  of  tlanges  and  numlier  of  rivets 
required,  the  r\iles  given  for  plate-girder  spans  are  to  apply 
also  to  stringers.  The  said  rules  are  to  apply  also  to  (mdss- 
girders,  as  shall  also  those  relating  to  stiffeners,  splices,  cover- 
plates,  and  size  of  compression- flanges,  that  are  given  for 
plate-girder  spans.  Wherever  it  is  necessary  to  notch  out  the 
corners  of  the  cross-girders  to  clear  the  chords,  the  greatest 
care  must  be  taken  to  provide  an  adeipiate  means  for  transfei- 
ring  the  shear  to  the  jxtsls  without  im[)airing  either  the 
strength  or  the  rigidity.  If  necessary,  in  through-bridges  the 
web  of  the  cross-girder  can  be  divided  into  three  parts  so  as 
to  let  the  end  portions  project  above  the  top  llange  and  form 
brackets  that  will  afford  opi)ortunity  for  using  an  ample 
ninuber  of  rivets  to  conne(;t  to  the  posts,  and  will  .strengthen 
P'operly  the  otherwise  weakeneil  cro-ss-girder. 

In  order  to  carry  the.  thrust  of  trains  from  the  stringers  to 
the  posts  through  the  lower  lateral  diagonals,  the  latter  and 
the  stringers  aru  to  be  madt;  to  form  complete  hoiizontal 
trusses  by  running  angles  between  stringers  at  the  leve.  of  the 
bottom  tlanges.  In  single-track  bridges  two  piec(;s  of  angles 
per  iiaiu'l  running  transversely  between  siringersat  the  iuter- 
iiecition  of  \hr  latter  niib  the  diagonals  will  sufflce;    but  in 


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174 


DE   PONTIBUS. 


doiible-tmck  bildgt'S  tlierc  will  be  n-qtiired  two  such  nngles 
per  pttiiel  belweeu  inner  stringers,  and  four  diiigonal  angles 
pe.  panel  to  run  from  where  the  Intend  diagonalH  intersect 
the  outer  stringers  to  whore  the  inner  stringers  nieot  the  crttes- 
girders. 

AH  plates,  angles,  and  channels  used  in  built  members  of 
trusses  must,  if  practicable,  be  ordered  the  full  length  of  the 
member;  otherwise  the  splices  must  develop  the  full  strength 
of  the  member,  without  any  reliance  being  placed  on  the 
abutting  ends  for  carrying  corapre&sion. 

But.  in  total  splices  at  the  ends  of  sections  perfect  abutting 
of  the  dix'ssed  ends  is  to  be  relied  upon.  However,  the  splice- 
platis  even  there  must  be  of  ample  size  and  strength  for  both 
rigidity  and  continuity. 

Tlie  unsupported  width  of  plates  strained  in  compression, 
measuring  between  centre  lines  of  rivets,  shall  not  exceed 
thirty-two  (33)  times  their  thickness,  except  in  the  CMse  of 
cover-plates  for  top  chords  and  inclined  end  posts,  where  the 
limit  may  be  increased  to  forty  (40)  times  the  ihickiHjss. 
Where  webs  arc  built  of  two  or  more  thicknesses  of  plate,  the 
rivets  that  are  used  solely  for  making  the  several  thicknesses 
act  as  one  plate  shall  in  no  ca8<!  be  spaced  more  than  twelve 
(12)  inches  from  each  other  or  from  other  rivets  connecting 
said  component  thicknesses  together.  The  least  allowable 
thickness  for  such  compound  web-plates  shall  be  one  (1)  inch. 

The  oi>en  sides  of  all  compression-members  composed  of 
two  rolled  or  built  channels,  with  or  without  a  cover-plate, 
shall  be  stayed  by  tie  plates  at  ends  and  by  diagonal  lacing- 
bars  or  lacing-angles  at  intermediate  points.  Lacing-lmrs  may 
be  connected  to  the  flanges  by  either  one  or  two  rivets  at  each 
end;  but  lacing-angles,  which  are  used  for  members  of  heuvy 
section  only,  must  be  connected  by  two  rivets  at  each  end. 

The  tie-plates  shall  be  placed  as  close  as  practicable  to  tiie 
ends  of  the  compression-members.  Their  thickness  shall  net 
be  less  than  one- fiftieth  (7('|;)of  the  distance  between  t!ie  centre 
lines  of  the  rivets  by  which  they  are  connected  to  the  flanges, 
unless  said  tie-plates  ])e  well  stiffened  by  angles,  in  widdi  case 
they  uiay  be  made  as  thiu  h9  three  eighths  (|)  of  an  inch. 


LACINO    AND    PIN-PLATES 


175 


The  Icugth  of  a  tie-plate  shiill  never  be  less  Ihiiu  its  width,  or 
one  and  one-half  (IJ)  times  the  least  dimension  of  strut  (unless 
it  be  close  to  a  web  diapiiragm  of  tlie  member,  in  which  case 
it  may  be  as  short  as  twelve  (12)  inches),  antl  seldom  greater 
than  one  and  one-half  (U)  times  its  width. 

The  thicknesses  of  lacing-bars  shall  nevei  be  less  than  one 
liflielh  (j'o)  of  the  Knglli  between  centres  of  the  end  rivets, 
nuasuring  between  inniost  rivets  in  case  that  there  be  more 
than  one  rivet  at  each  end.  The  smallest  section  for  a  lacing- 
bar  shall  be  one  and  three  quarter  (1  J)  inches  by  three  eighths 
(j)  of  an  inch,  which  size  shall  b;;  used  for  channels  under 
nine  (9)  inches  deep;  and  the  largest  section  shall  be  two  and  a 
half  (2A)  inches  by  one-half  ( j)  inch,  which  size  shall  be  used 
for  channels  lifteen  (15)  inches  deep.  For  intermediate  sizes 
of  channels,  the  sizes  of  laeing-btirs  shall  be  inlerpolaied.  For 
all  built  channels  of  greater  depth  than  fifteen  (15)  inches,  and 
for  nil  cases  where  a  lacing-bar  would  require  a  greater  thick- 
ness than  one-half  (j)  inch,  angle  lacing  is  to  be  used,  the 
smallest  section  for  same  being  3"  X  2i  '  X  I",  and  the  largest 
2i"  X  3*"  X  f  '.  For  two  (3)  inch  lacing-bars  and  two  and 
a  half  (2i)  inch  lacing  angles,  three-quarter  (J)  inch  rivets  are 
to  l)e  used;  and  for  two  nad  a  half  (2A)  inch  lacing-bars  and 
three  (3)  inch  lacing-angles  seven-eighths  (|)  inch  rivets  are 
to  be  adopted. 

In  general,  the  inclination  of  lacing-bars  to  axis  of  member 
shall  be  about  sixty  (60)  degrees ;  but  in  meml)ers  of  minor 
importance  and  in  tension- members  the  said  inclination  may 
be  made  slightly  Hatter. 

Pin-plates  shall  be  used  at  all  pinholes  in  built  members  for 
the  double  purpose  of  reinforcing  for  the  metal  cut  away  and 
reducing  the  intensity  of  pressure  on  pin  ami  bearing  to  or 
below  the  specified  limit.  They  shall  be  of  such  size  as  to 
distribute  properly,  through  the  rivets,  the  pressure  carried 
l)y  such  plates  to  both  tlanges  and  web  of  each  segment  of  the 
member  ;  and  tliey  shall  extend  at  l"*"^*  six  (6)  inches  within 
the  tie-plates  of  said  member,  so  as  to  provide  for  not  less  than 
two  (2)  transver.se  rows  of  rivets  there. 

When  the  pin  ends  of  compression -members  are  cut  awaj 


176 


DE   P0NT1BU8. 


into  jaw-plates  or  forked  ends,  for  the  purpose  of  packing 
closely  the  various  nieuibers  connected  by  the  pin,  tLw'se  juw 
plates  or  post  extensions  slmll  be  considered  as  columns,  the 
thickness  of  each  of  which  shall  be  determined  by  the  follow- 
ing formula : 

p=  10.000-300;; 

where  p  is  the  greatest  nllowable  intensity  of  working-stress 
(impact  being  consideied) ;  I  is  the  unsupported  Iciiglli  in 
inclies,  measuring  from  llie  centre  of  the  pinhole  to  the 
centre  of  the  hist  transverse  line  of  rivets  beyond  the  point  at 
which  the  full  section  of  the  member  begins  ;  and  t  is  the  tottd 
thickness  in  inches  of  one  jaw.  The  length  I  is  alwiiys  to  be 
made  as  small  as  pnicticnbU';  and,  iu  cases  of  unavoidably 
long  extensions,  the  plates  are  to  be  stiffened  by  an  interior 
diaphragm  composed  of  a  web  wiili  four,  or  sometimes  only 
two,  angles. 

It  is  always  better,  whenever  practicable,  to  avoid  cutting 
away  the  ends  of  channels  ;  but,  if  Ihey  must  be  trimmed,  the 
ends  must  be  reinforced  so  that  the  strength  of  the  member 
shall  not  be  reduced  by  the  trimming. 

In  riveted  tension-members,  the  net  section  through  any 
pinhole  shall  have  an  area  fifty  (50)  per  cent  in  excess  of  the 
net  sectional  area  of  the  body  of  the  member.  Tlie  net  sec- 
tion outside  of  the  pinhole  along  the  centre  line  of  stress  shall 
be  at  least  sixty-five  (65)  per  cent  of  the  net  section  through 
the  pinhole. 

Pins  are  to  be  proportioned  to  resist  the  greatest  shearing 
and  bending  produced  in  them  by  the  bars  or  struts  which 
they  connect.  No  pin  is  to  have  a  diameter  less  than  eight 
tenths  ( f%)  of  the  depth  of  the  deepest  eye-bar  coupled  thereon. 
No  truss-pin  is  to  have  u  smaller  diameter  than  three  and  a 
half  (8^)  inches,  and  no  lateral  pin,  if  any  such  be  used,  a 
diameter  less  than  two  and  a  half  (2i)  inches. 

Lower  chords  are  to  be  packed  as  closely  as  possible,  and  in 
such  a  manner  as  to  produce  the  least  bending  moments  on  the 
pins;  but  adjucfut  e3'e-bAis  iu  the  same  panel  must  never  have 


to 
pi-( 


CHORD-PACKING,    ROLLERS    AND    I'KDK.srALS.     177 


less  than  a  ODe  half  (i)  inch  space  between  them,  iu  ordei-  to 
facilitate 'painting.  The  various  menihers  attached  to  any  piu 
must  be  packed  as  closely  as  piiiclicable,  and  all  interior 
vacant  spaces  ni'ist  be  filled  with  steel  tillers,  wliere  their 
omission  would  permit  of  motion  of  any  member  ou  the  pin. 
All  bars  are  to  lie  in  plnnes  as  nearly  as  pfwsilJle  parallel  to 
the  central  truss-plane,  no  divergence  exceeding  one  eighth 
(J)  of  an  inch  to  the  fool  being  permitted. 

In  detailing  I  struts  composid  of  four  angles  with  a  single 
lino  of  lacing,  the  clear  distance  between  backs  of  angles  shall 
never  be  made  less  than  three-quarters  (J)  of  an  inch,  in  order 
to  permit  the  insertion  of  a  small  paint-brush. 

Th*;  greatest  allowable  pressure  upon  expansion-rollers  of 
lixed  spans,  when  impact  is  considered,  shall  be  determined 
by  the  equation 

p  =  600rf, 

■where  p  is  the  permissible  pressure  in  pounds  per  lineal 
inch  of  roller,  and  d  is  the  diameter  of  the  latter  in  inches. 
The  least  allowable  diameter  for  expansion-rollers  is  three  (3) 
inches. 

Rollers  shall  be  enclosed  in  boxes  made  practically  dust- 
tight,  but  which  will  not  retain  water,  and  which  are  so 
•leslgned  that  the  sides  can  be  readily  removed  for  the  purpose 
of  cleaning.  These  boxes  must  be  so  designed  as  to  permit  of 
a  free  movement  of  the  rollers  in  the  longitudinal  direction  of 
span  sufficient  to  take  up  the  extreme  variations  in  length  due 
to  temperature  changes  and  deflection,  and  at  the  same  time 
prevent  any  transverse  motion  of  the  end  of  the  span. 

All  sboe-plates,  bed-plates,  and  roller-plates  are  to  be  so 
stiffened  that  the  extren.e  fibre-stress  imder  bending,  when  im- 
pact is  included,  shall  not  exceed  sixteen  thousand  (16,000) 
pounds. 

Pedestals  shall  be  either  of  cast  steel  or  built  up  of  ulates 
and  shapes.  In  built  pedestals,  all  bearing  surfaces  of  the 
base-plates  and  vertical  bearing-plates  must  be  planed.  The 
vertical  plates  must  be  secured  to  the  base  by  angles  having 
at  least  two  rows  of  rivets  in  the  vertical  legs  ;  and  the  said 


178 


DK    I'ONJIHUS. 


vertical  plates  must  bear  properly  from  cud  to  end  upon  the 
base.  No  buse-plule,  vertical  plate,  or  coiiiiuctiiig  angle 
shall  be  less  in  thickness  than  three  quarters  (J)  of  an  inch. 
The  vertical  plates  shall  be  of  sufHcient  height  and  must  con- 
tain enough  metal  and  rivets  to  distribute  properly  the  loads 
over  the  bearings  or  rollers.  The  bases  of  all  casl-steel  i)ede8- 
tals  shall  be  planed  so  as  to  bear  properly  on  the  masonry  or 
rollers.  All  rollers  antl  the  faces  of  base-plates  in  contact 
therewith  are  to  be  planed  smooth,  so  as  to  furnish  perfect 
contact  between  rollers  and  plates  throughout  their  entire 
length. 

All  pedestals  whether  built  or  cast  must  have  one  or  more 
diaphragms  between  webs,  carried  up  as  high  as  the  general 
detailing  will  i)ermil,  .so  as  to  liansmit  transverse  horizontal 
thrust  to  the  base  without  overstraining  the  webs  by  bending 
in  their  weakest  direction. 

Heads  of  ( ye-bars  are  to  bo  made  of  such  dimensions  that 
when  the  bars  are  tested  to  destruction  they  shall  bieak  in  the; 
body  and  not  in  the  eyes;  and  in  case  of  lo()|)-eyes,  so  that 
they  shall  not  fail  in  the  welds.  Rods  with  bent  eyrs  shall 
not  be  used.  In  loop-eyes,  the  distance  from  the  innc  point 
of  the  loop  to  the  centre  of  the  pinhole  nuist  not  be  less  thnn 
two  and  one  h;ilf  (3i)  times  the  diameter  of  the  pin.  and 
the  loop  must  lit  closely  to  the  pin  throughout  its  entire  semi- 
circumference. 


DETAILS  OF  DESIGN  FOR  TRESTLES  AND  ELB- 
VATED  RAILROADS. 

The  sections  of  main  members  of  trestles  shall,  generally, 
be  as  follows;  Columns,  two  channels  laced  with  Uanges 
turned  either  out  or  in,  two  channels  with  I-beam  web 
between,  four  Z  bars  with  web-plate,  four  Z  bars  with  a 
single  line  of  lacing  inside  and  occasional  stay-plates  outside, 
or  four  angles  with  a  single  line  of  lacing  inside  ,  diagonals  in 
transverse  and  longitudinal  bracing,  and  nil  bottom  hori/.ontal 
bracing  struts,  four  angles  with  a  single  line  of  lacing; 
horizontal  transverse  bracing  struts  al  top  of  lowers,  bracing 


DKTAILS    OF   TRESTLES. 


179 


frames  of  tiuglus  ;  luiigitudiual  slnits  ut  top  of  towers,  plate 
girders ;  aud  longitudiiml  girders,  plate-girder  spans,  or 
occasionnlly,  for  very  long  spans,  open-webbed,  riveted 
girders  or  pin-conuccled  trusses. 

The  detailing  for  longitiidiuiil  girders  of  trestles  and 
elevated  railroads  and  the  bracing  between  sanic  shall  comply 
with  the  specifications  governing  the  designing  of  plutc- 
girder  spans  and  the  floor  systems  of  pin-connected  spans. 

In  '/uueral,  the  transverse  tind  longitudinal  bracing  of 
trestle  >wers  shall  consist  of  a  (loiible-cancellation  system  of 
stiff  dia.  >nals  without  auy  horizontal  struts,  except  at  the 
bottom  bc-lween  pedestals.  The  latter  struts  must  be  strong 
enough  to  move  the  column  feet  upon  their  sliding-bearings 
when  said  struts  are  expanded  or  contracted  by  (changes  of 
temiierature.  Provision  must  l)e  made  for  holding  some  feet 
rigidly,  and  for  sliding  some  in  one  horizontal  direction  only, 
and  others  in  an}'  horizortal  direction,  at  the  same  time 
holding  them  all  down  so  that  they  shall  not  be  lifted 
jK'rceptibly  by  the  wind-pressure.  Sliding-plates  are  always 
j)referal)le  to  rollers  for  pedestals  of  trestles.  They  shall  be 
planed  extremely  smooth,  and  so  as  to  bear  properly  at  aii 
parts. 

Occasionally,  in  solitary  bents,  it  is  permissinie  to  use 
hinged  ends  for  columns  at  pedestals ;  but  it  is  generally 
better  to  nuike  them  fixed,  and  to  figure  the  columns  for  the 
greatest  bending  produced  in  them  by  transverse  loads  and 
extreme  changes  of  temperature. 

The  tops  of  trestle  columns  are  to  be  made  vertical  by 
bending  them  just  beneath  the  longitudinal  girders  where  the 
latter  are  riveted  to  them  ;  and  the  upi)er  transverse  struts 
must  be  made  as  deep  as  the  longitudinal  girders,  and  must  be 
riveted  effectively  to  the  columns.  Corner  biackets  of  double 
webs  are  to  be  used  for  connecting  the  columns  to  the 
horizontiil  struts  and  bracing-diagonals,  and  at  the  same  time 
to  strengthen  the  column  at  the  bend.  Additional  strength- 
ening is  to  be  gi>v'n  by  using  a  solid  web  or  diaphragm  in 
the  column  extending  from  (he  top  thereof  to  a  point  about 
two  (3  feel  btlow  the  bend. 


ISO 


I)K    i'ONTlRl'S. 


All  Splices  lu  cultiinns  twc  to  bo  full,  butt  splices,  located 
preferably  about  two  ('J)feft  above  the  poiuJs  where  the  sway- 
diagonals  connect,  shiuglcsplicing  being  avoided  because  of 
the  trouble  it  gives  during  erection. 

The  best  span  lengths  for  trestles  are  generally  those  which 
make  the  total  cost  of  structure  a  minimum,  the  tower 
length  varying  from  twenty  (20)  feet  for  low  trestles  to 
thirty  (80)  feet  for  very  high  ones,  and  the  intermediate  spans 
varying  from  thirty  (30)  to  sixty  (60)  feet  for  the  same  limiting 
heights.  Any  length  of  girder  exceeding  sixty  (60)  feet  would 
probably  necessitate  the  employment  of  a  too  long,  heavy,  nnd 
expensive  traveller,  or  else  the  use  of  bents  of  falsework 
between  the  towers. 

For  elevated  railroads  the  sections  of  main  members  shall 
he  as  follows  :  Longitudinal  girders,  preferably  plute  girders, 
or,  if  necessary,  open-webbed,  riveted  girders;  cross- girders, 
plate  girders  ;  columns  for  structures  without  longitudinal  or 
tower  bracing,  two  channels  with  an  I  beam  riveted  between  ; 
and  columns  for  structures  with  longitudinal  or  tower  bracing, 
four  Z  bars  with  ii  web-plate. 

All  columns  for  elevated  railroads  are  to  have  both  ends 
fixed,  being  held  rigidly  at  tl:?  top  by  either  the  longitudinal 
girders  or  by  deep  struts  that  carry  the  thrust  of  braked  trains 
from  the  track  to  the  columns,  and  their  sectional  aieas  arc  to 
be  figured  accordingly  for  both  direct  innd  and  bending. 

Longitudinal  girders  in  elevated  railroads  shall,  generally, 
be  riveted  into  the  cross-girders  and  not  rest  thereon,  except 
under  certain  conditions  for  the  sake  of  clearance  beneath,  in 
which  case  the  top  flanges  of  the  half-through  girders  must  be 
stayed  at  the  ends  and  at  intermediate  points,  as  specified  for 
plate-girder  spans 

On  all  curves  in  elevated  railroads,  special  lateral  bracing  of 
angles,  riveted  at  intersections  to  the  longitudinal  girders  and 
carried  over  and  riveted  to  the  columns,  must  be  employed. 

Where  brackets  for  columns  can  be  used  advantageously  in 
elevated-railroad  work,  they  must  be  put  in,  and  must  be  built 
of  solid  web  plates  and  angles. 

In  general,  the  limiting  length  of  structure  between  expao-r 


DKTAIL8  OP   ELEVATED   RAILUOADS. 


181 


stoD  points  shall  be  about  one  hinulrcd  and  fifty  (150)  feet.  If 
Ibis  length  be  exceeded  materially,  the  culunins  may  have  to 
be  strengthened  to  resist  the  bending  caused  by  changes  in 
temperature. 

All  expansion-pockets  are  to  be  so  detailed  as  to  throw  the 
loitd  from  the  longitudinal  girder  as  close  as  possible  to  the 
web  of  the  cross-girder;  and  sufficient  rivets  are  to  be  used  in 
connecting  the  pocket  to  the  cross-girder  to  provide  for  l)oth 
the  direct  shear  and  the  bending  moment  from  the  eccentric 
load. 

All  anchor-bolts  at  column  feet  are  to  extend  well  up  above 
the  base-plate,  passing  inside  of  a  curved  plate  that  is  riveted 
to  the  column,  and  which  siipports  a  heavy  washer-plate  to 
leceive  the  anchor-bolt  nut.  The  space  between  the  curved 
plate  and  the  anchor-l)olt  aftei  erection  is  to  be  filled  with 
Portland-cement  grouting. 

All  column  feet  are  to  be  raised  so  far  above  the  ground 
that  no  dirt,  snow,  or  moisture  can  collect  around  them  and 
remain  there.  The  boxed  spaces  at  column  feet  are  to  be  filled 
wiih  Portlund-ceuient  coiirrele  made  with  small  broken  stone. 

The  bases  of  pedestals  are  always  to  be  made  large  enough 
to  prevent  all  possibility  of  settlement  of  foundations.  In 
figuring  Die  pressure  on  tlic  base  of  I  he  pedestals  it  is  not  suf- 
ticient  to  recognize  only  the  direct  live  and  dead  loads,  but  it 
is  necessary  also  to  compute  the  additional  unequal  intensi- 
ties of  loading  caused  by  both  longitudinal  and  transverse 
thrusts. 


CHAPTER  XV. 

SPECIFICATIONS   FOR  KAILROAI)   DRAW-SPANS. 

Thk  speciticRtions  given  in  the  preceding  chapter  for  fixed 
spans  apply  also  tu  draw-spuus,  except  where  otherwise  stated 
in  the  following  pages. 


GENERAL   DESCRIFTIOX. 
MATERIALS. 

The  speciflciitious  previously  given  apply  also  lo  draw- 
sp.ins,  except  tliiit  cast  iron  nmy  be  used  for  the  centre  cast- 
ings on  top  of  pivot-piers,  for  anchor- pieces  in  the  masonry, 
for  shafting  boxes,  and  for  ritiUchairs,  and  some  other  castings 
of  minor  importance.  Tlie  use  of  Idgh  steel  for  drawbridges 
will  not  be  permitted. 

8TT;,ES   ok  BRID(}KS   KOH    various    span   LEN0TH8. 

For  spans  up  lo  one  hundred  and  sixty  (160)  feet  in  length, 
plate-girder  spans  should  be  u>-e<l.  These  may  be  made  to  act 
as  continuous  girders  over  tlie  pivol-pier,  or  may  have  pin- 
connections  over  the  drum,  so  that  when  the  live-load  is  ap- 
plied they  will  act  as  two  separate  spans.  The  latter  style  is 
generally  preferable,  because  there  is  no  tendency  for  the  far 
end  of  the  span  to  rise  when  the  live  load  is  being  brought  on. 

For  spans  between  one  liundred  and  sixty  (160)  feet  and  two 
hundred  and  seventy-five  (275)  feet,  pin-connected  Pratt 
trusses  with  parallel  top  chords  and  stiff  diagonals  in  panels 
where  there  is  reversion  of  stress,  or  riveted  trusses  of  single 
cancellation,  are  to  be  used. 

For  spans  between  two  hundred  and  seventy-tive  (375)  feet 

188 


SPKCIFK  ATIONS    FOR   UAILUOAU    DKAW-S PAN'S.    183 


and  tliree  hundred  iind  flily  (:150)  feet,  piii-conntctod  Pratt 
iriisses  wilh  broken  top  diords  are  to  be  employed. 

For  8|vui8  of  over  three  hundrud  and  fifty  (850)  feet,  plii- 
connected  trusses  with  subdivided  panels  are  to  be  adopted. 

It  is  understood  tliat  tliesc  limiting  lengtiis  are  not  flxed 
abM)lulely,  as  tlie  best  liuiils  will  vary  somewhat  with  the 
number  yf  tricks  and  the  weight  of  triins. 

Tlie  height  of  towers  should  generally  be  between  one  sixth 
(l)  and  one  seventh  (\)  of  the  total  length  of  span,  measuring 
from  centre  to  centre  of  end>pins;  althougli  in  certain  cases  it 
may,  for  the  sake  of  appeamnce,  be  made  a  little  greater.  The 
truss  depth  at,  the  inner  lups  should  be  from  one  ninth  (^)  to 
one  tenth  (j'^)  of  the  total  length  of  span.  The  truss  depth  at 
outer  hips  for  spans  up  to  foijr  hundred  (400)  feet  will  gener- 
ally be  determined  by  the  clearance  required.  For  longer 
spans  it  should  be  between  one  fourteenth  {^)  and  one 
lifteenth  ( ,'j)  of  the  total  span  length. 

The  length  of  the  centre  panel  will,  in  most  ctises,  bo  made 
equal  to  the  perpendicular  distance  between  <enlral  planes  of 
trusses. 

In  spans  having  horizontal  top  chords  all  panels  of  the 
latter  must  be  made  of  stiff  members,  excepting  only  the 
centre  panel  over  the  pivol-pier;  and  thediugonals  next  to  the 
middle  panel  are  to  be  tension-members. 

Broken  top  chords  inust  be  made  of  stiil  members  from 
ends  to  inner  hips,  but  the  portion  between  the  inner  hips  is 
to  be  made  of  eye-bars.  Inclineil  posts  extending  from  inner 
hips  to  drum  are  to  be  used  in  all  cases  where  top  chords  are 
broken. 

LOADS. 

The  loads  to  be  considered  in  designing  draw-spans  are  the 
following : 
A.  Live  Load. 
13.  Impact  Allowance  Load. 

C.  Dead  Load. 

D.  Uplift  at  Ends. 

E.  Direct  Wind  Load. 

F.  Indirect  Wind  Load  or  Transferred  Load. 


184 


1)K    I'ONTIBUS. 


LIVK   liOADR. 

The  live  loads  for  the  various  parts  of  Uic  structure  are  to 
be  taken  from  the  "  Com  promise  Stuiulard  System  of  Live 
Loads  for  Railway  Bridges,"  iu  the  same  manner  as  previously 
specified  for  fixed  spans. 

Tlie  live  load  for  trusses  with  only  one  arm  loaded  Is  to  be 
taken  from  the  live-load  curves  for  a  span  equal  to  the  distance 
between  the  cent,  of  the  end-pin  tiud  that  of  the  pin  at  the 
foot  of  the  uearei  lower  post;  but  for  both  arms  loaded  the 
live  load  is  to  be  taken  for  a  span  equal  to  the  distance  between 
centres  of  end-pins. 

1  or  only  one  arm  loaded,  Ihu  half-span  is  to  be  considered 
to  act  as  a  simple  span  on  two  supports;  and,  for  both  arms 
loaded,  the  entire  span  is  to  be  considered  continuous  over 
four  supports.  The  stresses  due  to  the  live  load,  with  both 
arms  wliolly  or  partially  loaded,  are  to  be  determined  by  the 
balanced-load  method.  For  convenience  in  delcrmioing  the 
reactions  at  ends  and  at  centre  supports  for  balanced  loads  the 
curve  given  on  Plate  IX  can  be  used.  This  gives  the  per- 
centage of  any  balanced  load  which  is  supported  at  the  outer 
end  of  a  half-span. 

DEAD   LOADS. 

In  spans  over  two  hundred  and  seventy-five  (275)  feet,  the 
dead  load  per  truss  is  to  be  increased  properly  from  the  ends 
towards  the  centre  of  span  in  order  to  cover  the  weight  of  the 
heavy  truss-members,  which  increase  in  size  toward  the  centre 
of  the  span.  The  division  of  the  dead  load  between  top  and 
bottom  chords  is  to  be  the  same  as  specified  for  fixed  spans. 

The  dead  loads  from  tower,  drum,  and  turntable  arc  not  to 
be  considered  ns  affecting  the  stresses  in  the  trusses. 


ASSUMED   UPLIFT   LOADS. 


There  will  be  a  cousidcnvble  uplift  at  the  ends  of  the  span,  for 
luey  are  to  be  brought  to  a  firm  bearing  by  means  of  the  end- 
lifling  device.  The  amount  of  this  uplift  per  truss  or  girder 
is  to  be  assumed  as  a  certain  proportion  of  the  entire  deff  '  ^>ad 


8l'KCIKICATr()N'S    I'OU    UAILUOAD    DHAW-Sl'ANS.    185 

carried  by  one  arm  of  the  aaUl  truss  or  girder  wbeu  tliu  span 
is  being  swung,  which  proporiiun  is  to  be  tul^en  from  tlie  fol- 
lowing  table: 


SpatiH. 

Up  to  150., 
150' to  250'. 
250'  to  350'. 
850  to  450'. 
Over  450'.. 


Ration  of  U|)lift 
to  bfitd  Load. 


These  uplifts  are  to  be  adopted  both  for  finuinp;  the  uplift 
stresses  in  trusses  and  for  proportioning  the  ctid-liftiug  mu- 
chinery;  provided,  however,  that  for  the  latter  purpose  no 
assumed  uplift  1)e  less  than  twenty  thous!<nd  (20,000)  pounds 
for  single-trnck  drawbridges  or  less  ihun  forty  thousand 
(40,000)  pounds  for  double-track  drawbridges. 

WIND   1.0AD8. 

The  wind  loads  per  lineal  fool  of  span  for  l)oth  the  loaded 
and  the  unloaded  chords  arc  to  be  the  same  ns  those  specified 
for  fixed  spans,  the  length  of  span,  however,  being  that  of 
one  arm  of  tlie  draw. 

When  the  spun  is  open,  sill  the  wind  load  is  to  be  carried  to 
the  drum  througli  the  latend  systems.  Wiien  tlie  draw  is 
closed,  the  wind  load  is  to  be  carried  to  both  tlie  ends  and 
the  centre  supports,  the  lower  lateral  system  and  bottom 
chords  being  considered  to  act  as  a  continuous  girder  over 
four  supports.  The  reactions  at  the  ends  and  the  centre  can 
be  taken  from  the  curve  for  balanced  live  loads. 


INDIRECT   WIND   LOAD  OR   TRANSFERRED   LOAD. 

The  wind  load  on  the  upper  chords  is  to  bo  assumed  to 
travel  through  the  upper  lateral  system  to  the  inner  hips, 
when  the  span  is  open,  then  down  the  inner  iucUned  posts  to 
the  drum,  thus  producing  a  transrerred  load  on  the  leeward 
inclined  post  and  a  released  load  on  the  windward  one.  As 
the  upper  lateral  system  is  not  continuous  between  the  inner 


186 


IJE    PONTFBUS. 


bips,  noue  of  the  wind  load  on  the  upper  lateral  system  is 
curried  down  the  tower-postn,  excepting  that  which  comes  ou 
the  centre  panel  and  the  two  adjacent  panels.  In  order  to 
ensure  such  a  distribution  of  the  wind  load  it  is  necessary  to 
put  no  diagonals  in  those  panels  of  the  upper  lateral  system 
which  are  adjacent  to  the  inner  hips  and  between  same  and 
the  tower. 

When  the  draw  is  closed,  one  half  of  the  wind  load  on  the 
upper  lateral  system  of  one  arm  is  to  be  assumed  to  travel 
down  the  end  inclined  posts,  and  one  half  down  the  inner 
inclined  posts. 

The  transferred-load  stress  on  an  inclined  ix)st  is  to  be 
found  by  multiplying  the  wind  load  going  to  it  by  the  aver- 
age height  of  the  lop-chord  panel  points  to  which  said  wind 
load  is  applied,  dividing  the  product  by  the  perpendicular 
distance  between  central  planes  of  trusses,  and  multiplying 
the  quotient  by  the  secant  of  the  angle  that  the  inclined  post 
makes  with  the  vertical. 

The  transferred- load  stress  on  a  tower-post  is  to  be  deter- 
mined by  multiplying  the  wind  loads  carried  by  the  two 
opposite  posts  by  the  respective  heights  at  which  these  loads 
are  applied,  and  dividing  the  sum  of  these  products  l)y  tiie 
perpendicular  distance  between  central  planes  of  trusses. 


COMBINATIONS    OF    STRESSES. 


In  ascertaining  the  stresses  in  the  trusses  of  swing-bridges 
the  following  conditions  are  to  be  considered  : 

Case  No.  1.  Greatest  stresses,  dead  load  only  acting,  bridge 
swinging  open. 

Case  No.  2.  Greatest  stresses  from  assumed  uplift  at  end 
of  span. 

Case  No.  3.  Greatest  stresses  from  live  load  ou  one  arm 
only  ;  each  arm  being  considered  to  act  as  a  simple  span  ou 
two  supports. 

Case  No.  4.  Greatest  stresses  from  live  load  on  both  arms, 
the  live  load  advancing  from  botit  ends  toward  the  centre 


SPECIFICATIONS  POK   KAILROAI)   DRAW-SPANS.    187 


until  the  span  is  fully  loaded  ;  the  hitter  being  considered  to 
act  as  a  continuous  girder  over  four  supports. 

Case  No.  5.  Greatest  direct  stresses,  on  the  chords  that  carry 
the  live  load,  from  wind  load  when  the  bridge  is  open. 

Case  No.  6.  Greatest  direct  stresses,  on  the  chords  that  carry 
the  live  load,  from  wind  load  when  the  bridge  is  closed  and 
wholly  or  partially  loaded. 

Case  No.  7.  Greatest  indirect  wind-load  stresses  or  trans- 
ferred-load stresses  on  the  lower  chords  when  the  bridge  is 
closed  and  wholly  or  partially  loaded. 

The  first  combination  of  these  stresses  includes  Cases  No. 
1,  No.  r.  No.  3,  and  No.  4,  and  gives  the  greatest  stresses  for 
all  tniss  members  from  combined  live  and  dead  loads,  for 
which  combination  the  regular  specitied  intensities  of  worli- 
ing-stresses  are  to  be  used.  It  is  to  be  noted  that  wherever 
the  load  for  Case  No.  3  increases  the  tolal  stress  on  any  mem- 
ber, its  effect  is  to  be  considered  ;  but  wherever  the  said  load 
decreases  the  total  stress  on  any  member,  its  effect  it?  to  be 
ignored.  The  reason  for  this  is  tlmt  the  amount  of  uplift  is 
a  purely  arbitrary  assiunption,  which  possibly  may  never  be 
realized.  This  method  of  treating  the  uplift-loud  stresses 
causes  errors  on  the  side  of  safety,  whleh  do  uol  ndd  materi- 
ally to  the  total  weight  of  metal  in  the  structure,  nnd  which 
tend  to  strengthen  the  lighter  members  of  the  trusses. 

The  second  combinati<ni  of  these  stresses  includes  all  seven 
eiises,  but  il  is  to  be  noticed  that  the  c  ,y  truss  members 
affect<;d  by  the  wind  loads  are  the  inclined  posts  at  ends  and 
over  drum,  and  the  cliords  which  carry  the  live  load.  In  this 
secoivd  combination  it  must  not  be  forgotten  that  the  metal  is 
to  be  strained  tlrirty  (30)  per  cent  higher  than  in  the  lirst 
combination, 

Fot  the  lateral  systems  the  following  conditions  are  to  be 
considered  : 

For  upper  lateral  systems  of  through-bridges  and  lower 
lateral  systems  of  deck-bridges — 

Cane  No.  1.  Greatest  wind  -  load  stresses  when  span  Is 
swinging. 

Case  No.  2.  Greatest  wind-load  stresses  wlien  span  is  closed 


188 


DE  rONTlBtfS. 


aud  euds  are  raised,  thus  makiug  the  CDtire  lower  lateral 
system  with  the  bottom  chords  a  continuous  girder  with  four 
points  of  support.  This  case  does  not  involve  the  presence 
of  any  live  load  on  the  span. 

For  lower  lateral  systems  of  through-bridges  and  upper 
lateral  systems  of  deck-bridges — 

Case  No.  3.  Greatest  wind-load  stresses  when  span  is 
swinging. 

Case  No.  4'  Greatest  wind  load  stresses  when  span  is  closed 
and  ends  are  raised,  and  with  live  load  on  one  arm  only,  tlius 
making  the  loaded  chords  with  their  lateral  system  a  simple 
span  with  supported  ends. 

Case  No.  5.  Greatest  wind-load  stresses  when  span  is  closed 
aud  euds  are  raised,  and  with  the  live  load  on  both  arms  cover- 
ing same  either  wholly  or  partially,  thus  makiug  the  loaded 
chords  with  their  lateral  system  a  continuous  girder  with  four 
(4)  points  of  support. 

The  greatest  stress  on  any  lateral  member  found  by  these 
live  conditions  of  wiud-Ioading  is  to  be  used  in  proportioning 
its  section,  and  there  is  to  be  assumed  no  division  of  the  wind 
load  between  structure  and  train,  although  the  failure  to 
make  said  division  will  cause  small  errors  on  the  side  of 
safety. 


DETAILS  OP  DESIGN  FOR  PIjATE-GIRDER 
DRAW-SPANS. 

Plate-girder  drawbridges  are  to  be  divided  into  two  types, 
viz.: 

Type  No.  1.  Continuous  girders,  in  which  the  girders  act  as 
continuous  spans  resting  on  four  points  of  support;  and 

Type  No.  2.  Non-continuous  girders,  in  which  the  two  arms 
carry  the  live  load  independently  of  each  other,  the  dead-load 
stresses  over  the  pivot  pier  when  the  span  is  swung  being 
carried  by  links. 

For  Type  No.  1  the  same  combinations  of  stresses  are  to  be 
used  as  specified  for  truss  draw-spans,  but  it  will  generally  be 
found  lluit  the  wind  loads  do  not  affect  the  proportioning  of 
the  girders. 


SPECIFICATIONS    FOR   RAILROAD    DRAW-Sl'ANS.    189 


For  Type  No.  3  the  loads  to  be  considered  nre  as  follows; 

Case  No.  1.  Dead-lodd  stresses  wlicii  the  span  is  swung. 

Case  No  2.  Dead-load  stresses  for  each  arm  acting  inde- 
]H-ndently  of  the  other. 

Case  No.  3.  Live-load  stresses  for  each  arm  acting  iude- 
l)endeutly  of  the  otiier. 

The  stresses  in  Cases  No.  3  and  No.  3  are  to  be  combined, 
but  those  in  Ca.sc  No.  1  are  not  to  be  combined  with  either  of 
the  otliers,  the  effect  of  reversion  of  stress,  however,  being 
provided  for  as  specified  for  fixed  spans. 

The  only  effect  of  wind  load  to  be  considered  for  the  girders 
of  Type  No.  3  is  that  upon  the  connecting  links  over  the  turn- 
table when  the  span  is  being  rotated,  for  which  (a.se  the 
amount  of  the  wind  load  is  to  be  taken  at  two  hundred  (200) 
I)onnds  per  lineal  foot  of  span. 

In  general,  the  specifications  for  the  detailing  of  fl.xed  plate- 
girder  spans  are  to  govern  the  designing  of  plate-girder  draw- 
spans,  except  as  hcniriifier  stated. 

In  deck,  plate-girder  draw-spans  the  girders  are  to  be  .«paced 
the  same  distance  apart  as  specified  for  fixed  plate-girder 
spans  of  one  half  the  length.  For  half-through,  plate-girder, 
draw-spans  the  girders  may  be  spaced  as  closely  as  the  pre- 
viously specified  clearance  requirements  will  permit. 

For  deck-spans  four  points  of  support  on  the  drum  will  suf- 
fice, but  for  half-through  spans  eight  points  will  be  required. 
Tlie  diameter  of  the  drum  is  to  be  made  as  small  as  pr;,<;ticabU', 
but  never  less  than  eight  (8)  feet;  and  the  distribution  of  the 
load  over  the  drum  is  to  be  uniform. 

All  girders  are  to  be  thoroughly  stiffened  at  all  points  of 
bearing  over  the  drum,  and  bearing-iilates  not  less  than  one 
(I)  inch  in  thickness  are  to  be  used  between  the  drum  and 
all  girders  bearing  on  same. 

For  spans  of  Type  1,  when  the  length  over  all  exceeds 
ninety  (90)  or  at  the  utmost  one  hundred  (100)  feet,  it  will  be 
necessary  to  splice  the  main  girders  in  the  field.  These  splices 
must  be  thoroughly  made,  shingle  or  staggered  splices  only 
being  allowed;  and  there  must  be  a  tweuty-flve  (25)  per  cent 


190 


DE    POSTIBUS. 


excess  of  strength  in  the  deltiils  at  all  points  thus  spliced,  iis 
previously  specilicd  for  lixed  pliitc-glrdcr  spans. 

Iti^id  bracing- frames  are  to  be  useil  between  inuin  girders 
of  deck-spiins  at  the  points  where  the  main  girders  bear  en  ihe 
drum;  and  lie.ivy,  rigid,  i)latc  cross-girders  resiing  on  li.e 
drum  are  to  be  used  for  hnlf-througii  spans. 

End  lifts  must  be  provided  for  draw-spans  of  Type  No.  1, 
as  hereinafter  specified  for  truss-span  dravvbi  idges. 

For  spans  of  Type  No.  2  tlie  centre  panel  is  to  be  made  with 
pin  connections,  the  bottom-chord  pins  resting  in  pedestals, 
which  furnish  proper  bearings  on  the  drum.  The  top-chord 
ten.sion  is  to  be  taken  up  by  eye-bars,  which  serve  as  togglis 
for  raising  the  ends  of  span.  These  toggles  are  to  be  worked 
by  a  screw  at  centre  of  span. 

The  compression  in  bottom  flanges  of  girders,  due  to  dead 
load  when  the  span  is  swung,  is  to  be  taken  up  by  strut's 
hinged  on  the  bottom-chord  pins. 

The  eye-bars  of  the  top  chords  must  have  slotted  eyes,  so  as 
to  make  sure  that  each  half  of  the  girder  will  act  as  a  simple 
span  when  the  live  load  is  applied. 

Proper  shoes  must  be  provided  at  ends  of  spun,  wilii 
grooves  into  which  the  sole-plates  on  ends  of  girders  are 
loweied  into  place.  These  grooves  sho\dd  be  deep  enough  to 
hold  the  ends  of  the  girders  securely,  and  the  toggle  at  the 
centre  nmst  provide  enough  lift  to  clear  the  emls  properly  for 
turning. 

All  track-rails,  guard-rails,  and  stringers  must  be  discon- 
tinuous in  the  centre  panel  so  that  the  toggle  will  be  free  to 
act. 

The  ends  of  each  pair  of  girders  over  the  drum  must  be 
thoroughly  braced  together. 

The  end  lifting  arrangement  of  these  spans  demands  the 
most  accurate  sliop-work  ;  and  in  every  case  the  whole  span 
must  be  assembled  in  the  shops,  so  that  the  lifting  nuichinery 
can  be  thoroughly  tested  before  being  shipped. 


SPECIFICATIONS   FOR    IIAII.UOAI)    DKA W-SI'ANS.    191 


DETAILS  OP  DESIGN  FOR  TRUSSES. 

The  details  of  trusses  for  dniw  spans  siisill  comply  In 
general  with  the  specifications  given  for  trusses  of  fixed  spans 

In  trusses  liuving  broken  top  chords,  I  hat  portion  of  said 
top  chords  between  outer  and  inner  hips  is  to  be  made  of  rigid 
members,  and  that  portion  between  the  inner  hips  and  over 
the  tower  is  to  be  made  of  eye- bars. 

In  pin-connected  trusses  with  parallel  chords  rigid  mem- 
bers will  be  required  tiiroughout  the  top  chord,  except  for  the 
centre  panel,  in  which  eye-bars  are  to  be  used.  In  riveted 
trusses  stiff  top  chords  from  end  to  end  of  span  are  to  be 
adopted. 

The  bottom  chords  are  to  be  of  rigid  sections  throughout 
for  all  spans ;  and  for  spans  over  three  hundred  (300)  feet  in 
length  provision  must  be  made  near  the  panel  points  at  feet 
of  tower-posts  for  adjusting,  by  means  of  siiimming-plates, 
the  height  of  the  ends  of  the  trusses.  These  shimming-,.iitte3 
must  provide  an  end,  vertical  adjustment  of  one  (1)  inch  for 
each  one  hundred  (100)  feet  of  length  of  one  arm  of  draw. 
For  spans  shorter  than  three  liundred  (300)  feet  shimming- 
plates  beneath  the  end  bearings  will  give  sufficient  adjust- 
ment. 

Rigid  portal-bracing  must  be  used  between  the  two  in- 
clined posts  at  both  the  inner  and  the  outer  hips.  These 
portals  are  to  be  carried  down  as  low  as  the  specified  clearance 
over  tracks  will  permit. 

In  heavy  spans  the  portal-bracing  must  attach  to  the  upper 
and  lower  flanges  of  inclined  posts,  instead  of  lying  in  tlie 
gravity-planes  of  same. 

The  tower  must  be  rigidly  braced  in  all  four  faces.  In  the 
tmnsverso  planes  all  the  diagonals  and  horizontal  struts 
must,  generally,  be  niade  of  stiff  members  of  box  or  I  .sections, 
so  as  to  take  hold  of  the  exterior  of  the  posts  ;  and  this  sway- 
bracing  must  be  carried  down  as  low  as  the  specified  clearance 
will  permit,  so  as  to  hold  the  tower-posts  firmly  to  place  and 
line. 

In  tho  planes  of  the  trusses  the  diagonals  are  to  be  made  of 


192 


HE    I'ONTIHUS. 


adjustable  rods  of  ample  section  to  provide  for  any  possible 
unequal  vertical  wind-pressure  when  the  spaa  is  open  ;  and 
the  horizontal  struts  of  box  or  I  sections  are  to  be  rigidly  at- 
tached to  the  columns  by  large  plates,  to  which  the  clevises  of 
the  adjustable  rods  attach  by  means  of  pins. 

A  pair  of  adjustable  diagonal  rods  or  rigid  struts  must  be 
useil  in  the  horizontal  plane  of  each  vertical  panel  of  tower- 
bracing,  so  as  to  ensure  the  permanent  rectaugularily  of  the 
section  of  the  tower. 

All  splices  in  top  and  bottom  chords,  inclined  posts,  and 
tower-posts  are  to  be  full  splices,  so  as  to  develop  the  full 
strength  of  the  section,  even  if  the  computed  stresses  do  not 
demand  such  a  strength  of  detail. 

Tlie  upper  lateral  system  between  the  inner  and  the  outer 
liips  is  to  be  made  of  rigid  diagonals,  capable  of  taking  both 
lension  and  compression,  and  transverse  struts  of  I  section, 
that  take  firm  hold  of  the  upper  and  lower  flanges  of  the  top 
chords.  From  inner  hip  to  inner  liip  the  diagonals  are  to  be 
of  adjustable  rods  ;  but,  as  before  stated,  the  rods  are  to  be 
omitted  from  the  panels  next  to  tlie  hips,  so  as  to  ensure  a 
proper  travel  of  the  wind-loads  to  the  pivot-pier. 

The  transverse  sway-bmcing  between  trusses  is  to  be  made 
entirely  of  rigid  members,  and  is  to  be  carried  down  as  low  as 
clearance  requirements  will  permit.  In  long  spans  the  lower 
horizontal  struts  of  the  vertical  sway  bracing  must  take  liold 
of  the  vertical  posts  at  the  flanges  of  same,  so  as  to  hold  the 
said  posts  firmly  in  position. 


DETAILS  OP  DRUM   AND  TUBNTABLE. 

The  drum  must  be  strong  enough  to  distribute  the  total  load 
from  the  span  properly  over  the  rollers.  In  general,  it  should 
be  made,  within  reasonable  limits,  as  deep  as  possible,  for  the 
cost  for  the  extra  depth  will  be  more  than  offset  by  the  saving 
in  height  of  pivot-pier. 

The  bending  moment  on  the  drum  is  to  be  computed  by  the 
compromise  formula, 


SPECIFICATIONS   FOIl    KAILltOAJ)    DRAW-SPANS.    193 


where  Jf  =  beading  moment  in  foot-pounds,  W=  greatest 
load  in  pounds  on  one  point  of  bearing  on  drum,  and  I  =  dis- 
tance in  feet  between  points  of  bearing. 

The  drum  is  to  be  designed  according  -to  the  specifications 
for  ordinary  plate  girders.  The  web  thereof  siiall  have 
stiffeners  on  both  sides  at  all  points  of  concentraiion.  These 
stiffeners  must  have  perfect  contact  with  the  top  and  bottom 
flanges.  The  section  required  for  these  stiffeners  is  to  be  de- 
termined by  considering  the  entire  concentration  on  one  point 
of  bearing  to  be  carried  by  the  said  stiffeners,  which  act  as  a 
column,  fixed  at  both  ends,  with  an  unsupported  length  equal 
to  the  depth  of  drum.  Stiffeners,  each  consisting  of  two 
angles,  placed  on  opposite  sides  of  the  web  must  be  used  at 
intermediate  points  at  distances  not  exceeding  cither  the  depth 
of  web  or  three  (3)  feet  six  (0)  inches. 

Brackets  to  support  the  pinions  gearing  into  the  rack  are  to 
be  provided  on  the  drum.  They  shall  be  built  of  rolled-steel 
sections,  and  made  amply  strong  in  all  directions  and  in  every 
particular  so  as  to  resist  the  greatest  thrust,  wrenching,  or 
torsion  that  can  possibly  come  from  the  shaft.  In  no  case  are 
these  brackets  to  Imj  made  of  castings.  The  use  of  turned  bolts 
for  attaching  the  brackets  to  the  drum  will  not  he  permitted 
where  it  is  possible  to  drive  rivets,  as  such  bolts  do  not  afford 
sufiicient  rigidity  to  prevent  the  connections  from  working 
loose  sooner  or  later.  The  splices  in  the  web  and  flanges  of 
drum  must  be  such  as  to  develop  the  full  strength  of  same  ; 
and  the  abutting  ends  of  web  and  flanges  must  be  planed 
smooth,  and  have  continuous  contact. 

The  drum  must  be  made  perfectly  round,  so  that  the  centre 
lincof  web  at  any  height  will  conform  to  the  circumference  of 
a  circle;  and,  to  preserve  this  form  and  brace  the  drum  thor- 
oughly, rigid  radial  struts  are  to  be  run  from  the  centre  cast- 
ing to  the  drum,  taking  hold  of  the  latter  at  each  point  of 
concentrated  loading,  and  at  intermediate  points  when  the 
bearings  are  spaced  more  than  eight  (8)  feet  between  centres. 
These  radial  struts  must  be  made  of  four  angles  with  solid 
webs  or  angle  lacing.  At  the  cenire  they  are  to  be  riveted  to 
circular  plates  fitting  closely  around  ibe  centre  casting,  tljus 


194 


DE   P0NTIBU8. 


nnchoriiig  the  drum  firmly  to  the  latter.  Oil-grooves  must  be 
provided  where  these  plates  bear  on  the  centre  casting.  Fill- 
ers are  to  be  used  beneath  all  stifTeners  on  drum. 

The  drum  must  be  assembled  and  the  bottom  must  then  be 
planed  smooth  so  ivs  to  provide  an  even  bearing  for  the  upper 
track.  If  it  is  not  practici  ble  to  plane  the  entire  drum  at 
once,  then  each  segment  thereof  is  to  be  planed  separately  ; 
but  in  this  case  the  greatest  care  is  to  be  taken  to  make  the 
assembled  parts  form  a  perfect  whole. 

The  least  thickness  of  metal  to  ho  used  for  bottom  flanges  of 
drum  shall  be  three  quarters  (J)  of  an  inch,  so  as  to  provide 
ample  metal  for  planing  off  the  botiom,  and  that  for  the  web 
and  top  flanges  one-half  (^)  inch. 

The  upper  track  shall  be  made  of  segments  of  sufllcient 
thickness  to  distribute  the  load  properly  between  the  rollers 
and  the  drum.  The  top  face  of  this  tniek  shall  be  planed 
smooth  so  as  to  form  close  contacl  with  the  bottom  flange  of 
the  drum,  and  the  lower  face  shall  be  planed  conical  so  as  to 
tit  closely  to  the  conical  rollers.  All  joints  between  segments 
are  to  be  planed  smooth  and  to  such  bevel  as  to  ensure  perfect 
contact  with  each  other.  These  track  segments  are  to  be  riv- 
eted or  bolted  to  the  bottom  flanges  of  the  drum  with  fifteen- 
sixteenths  ([f )  inch  rivets  or  bolts,  placed  opposite,  and  spaced 
not  to  exceed  fifteen  (15)  inches  between  centres.  The  heads 
of  these  bolts  or  rivets  are  to  be  countersunk  in  the  track  on 
the  side  next  to  the  rollers. 

No  rust-cement  or  any  otiier  composition  is  to  be  used  be- 
tween the  track  and  the  drum. 

The  lower  track  is  to  made  strong  enough  to  distribute  the 
load  from  the  rollers  uniformly  over  the  masonry.  The  bend- 
ing moment  on  the  lower  track  is  to  be  found  by  the  formula 


where  M  —  greatest  bending  on  lower  track,  TT  =  total  load 
on  one  roller,  and  I  =  distance  fiom  centre  to  centre  of  adja- 
cent rollers,  measured  on  the  centre  line  of  the  tnick. 
The  greatest  allowable  tensile  stress  on  the  extreme  fibre  for 


SPECIFICATIONS   FOR   KAILUOAI)    DKAW-Sl'/.  N8.    105 


cust-sleel  truck  shall  uot  exceed  eight  thousand  (8,000)  pounds 
per  square  inch,  when  the  effect  of  impuct  is  included.  The 
lower  track  shall  be  made  in  segments  from  six  (6)  to  eight  (8) 
feet  in  length.  All  abutting  ends  of  lower-track  segments  are 
to  be  planed  smooth,  are  to  have  close  contact  throughout, 
and  arc  to  be  bolted  together  by  two  bolts  passing  through 
holes  in  lugs  cast  thereon.  These  bolts  are  to  be  at  least  fif- 
teen sixtcenttis  (|^>)  of  an  inch  in  diameter. 

In  no  case  shall  the  upper  truck  be  less  than  two  and  one- 
(piarter  (2J)  inches,  or  the  lower  truck  less  than  two  and  one- 
half  (2^)  inches  thick,  measuring  on  the  central  cylindrical 
surface  of  the  drum. 

Tl»e  lower  track  shall  be  anchored  to  the  top  of  the  pivot- 
pier  with  bolts  uot  less  than  one  (1)  inch  in  diameter,  nor  less 
than  lifteeu  (15)  inches  long,  set  in  place  with  Portland  cement 
grouting.  These  bolts  arc  to  be  made  of  soft  sicel,  with  cold- 
pressed  threads  and  hexagonal  nuts  at  top,  and  with  spMt  e'.ids 
and  wedges  at  the  bottom.  They  are  to  be  placed  in  pairs 
opposite  on  the  inside  and  outside  of  the  track,  and  are  to  be 
spaced  not  to  exceed  eighteen  (18)  inches  between  cenlres. 

Tlie  lop  of  the  pier  is  to  be  levelled  off  with  neat,  Portland 
cement  mortar,  and  the  lower  track  is  to  be  set  in  sjime.  It 
shall  be  made  one  and  one-half  (IJ)  or  two  (3)  inches  higher  in 
the  centre  than  at  the  edge,  so  that  the  water  will  drain  toward 
the  latter.  A  snuUl  gutter  or  depression  in  the  top  of  the  pier 
is  to  be  made  just  inside  of  the  lower  track,  and  at  the  bottom 
of  this  depression  drain-holes  are  to  bo  put  in,  leading  the 
water  from  the  gutter  down  ou  the  outside  of  the  pier.  These 
drain-holes  are  to  be  at  least  two  (3)  inches  in  diameter;  and 
the  tops  are  to  be  protected  with  screens,  so  as  to  prevent 
choking.  They  are  to  be  spaced  uot  to  exceed  ten  (10)  feet 
between  centres. 

The  rollers  shall  be  of  cast  steel,  and  are  to  be  ::<iade  solid, 
excepting  only  the  centre  hole  and  four  or  more  radial  holes 
that  are  left  in  the  casting  for  the  double  purpose  of  reducing 
the  weight  and  facilitating  a  rapid  and  uniform  cooling,  the 
said  holes  varying  in  size  and  number  with  the  diai^eter  of  tk*? 
roller. 


196 


DE    PONTinUS. 


The   fuiiowiiig   formiilte    Hliall   be  used   in   proportioning 

rollers ; 

For  greatest  total  londs,  iiicludiug   impact,  with  drtiw  at 

rest, 

p  =  mO(l; 

for  loads  with  dn»w  in  motion, 

p  =  200rf, 

•where  p  is  tiie  permissible  pressme  in  potmds  per  lineal  inch 
of  roller,  and  d  is  its  mean  diameter  in  inches. 

In  no  case  shall  the  roller  be  less  than  twelve  (12)  inches  in 
diameter  and  seven  (7)  inches  on  face. 

All  rollers,  and  the  faces  of  the  upper  and  lower  tracks 
which  are  in  c(mtHcl  with  the  rollers,  are  to  be  turned  smooth 
to  the  forms  of  riglit  frustums  of  cones,  the  vertices  of  whlcli 
intersect  at  the  centre  of  the  drum,  so  that  the  rollers  will 
have  perfect  contact  with  the  tracks  throughout  their  travel 
around  the  entire  circumference. 

A  bearing  is  to  be  turne  1  in  the  centre  of  each  roller  for  th  ; 
radial  rod,  and  oil-holes  are  to  be  provided  on  both  the  interior 
and  the  exterior  ends  of  the  rollers,  so  that  these  bearings  can 
be  kept  well  luljricated. 

The  outer  ends  of  the  radial  rods  are  to  pass  through  the 
rollers,  and  the  inner  ends  are  to  attach  to  a  circular  plate 
fitting  closely  around  the  centre  casting.  These  radial  rods 
are  to  be  provided  with  either  turnhuckles  or  nuts  for  adjust- 
ing the  position  of  the  rollers.  Only  square  sections  are  to  be 
used  for  the  rods,  and  each  must  contain  at  leaiit,  one  square 
inch  of  section.  The  end  of  the  rod  passing  through  the  roller 
must  be  upset  so  as  to  provide  a  tJirned  shaft  for  the  latter  at 
least  one  and  one-half  (li)  inches  in  diameter.  The  outer  ends 
of  these  rods  are  to  pass  through  a  stiff  steel  ring  of  rolled  ov 
built  channel  section,  which  is  to  serve  as  a  spacer  for  the 
rollers.  Tliese  channels  must  be  made  wide,  but  not  deep, 
and  their  section  is  to  be  commensurate  with  the  size  of  the 
turntable.  They  are  to  be  held  away  from  the  rollers  by 
friction -washers  on  the  rods. 

Pn  the  inside  of  ihp.  rollfirs  ciAhyn  syw  )o  ))e  forged  and 


9Pi;Cn'i('ATlox8 


FOR  RAILUOAD   DRAW-SPANS.    197 


turned  on  the  radial  rods  to  hold  the  said  rollers  in  exact 
position  on  same.  Turned  bosses  must  be  provided  on  both 
tite  inner  and  the  outer  ends  of  the  rollers,  to  bear  uguinst  the 
collars  and  the  friction- washers. 

An  inner  spacing-ring,  of  size  commensurate  with  the  mag- 
nitude of  the  drum,  is  to  bo  attached  lo  the  radial  rods.  For 
large  drums  this  should  be  in  the  form  of  a  small  curved 
plate  girder  lying  in  a  horizontal  plane  and  rigidly  braced  to 
tlie  centre  casting  by  radial  struts  that  are  riveted  at  the  outer 
ends  to  the  curved  girder  and  at  the  inner  ends  to  a  large 
circular  plate  which  lits  snugly  around  a  ttirned  bearing  on 
the  centre  casting.  With  this  detail  the  radial  rods  are  to  be 
dispensed  with,  and  in  their  stead  are  to  be  substituted  heavy 
square  bars,  having  their  outer  ends  detailed  as  described  for 
the  radial  rods,  and  their  inner  ends  attached  to  the  circidar 
girder  so  as  to  hold  the  bars  in  a  position  exactly  radial  to  the 
drum.  These  bars  should  not  be  less  than  two  and  a  half  (3A) 
inches  s(]uare,  and  the  journals  should  not  be  less  than  three 
(3)  inches  in  diameter.  There  must  be  nuts  at  both  ends  of 
the  bars  so  as  to  move  the  rollers  in  a  radial  direction,  and  the 
inner  ends  of  the  bars  are  to  be  so  attached  to  the  circular 
plate  as  to  permit  of  the  correction  of  any  slight  variation  of 
their  axes  from  a  truly  radial  direction. 

The  centre  casting  must  be  made  strong  and  heavy,  and 
mtist  be  effectively  anchored  to  the  top  of  pier  by  eight  (8)  or 
more  anchor-bolts  not  less  than  one  and  one-fourth  (li)inchea 
in  diameter  and  not  less  than  three  (3)  feet  long.  These 
bolts  are  to  be  made  of  soft  steel,  with  cold-pressed  threads 
and  hexagonal  nuts  at  top,  and  with  split  ends  and  wedges  at 
bottom.  The  least  allowable  thickness  of  metal  for  this  cast- 
ing shall  be  one  and  one-half  (1.J)  inches.  The  base  shall  be 
true  and  level;  and  an  even  bearing  shall  be  secured  by  bed- 
ding in  neat,  Portland-cement  morlar.  For  heavy  draws  this 
centre  casting  is  to  be  set  well  into  the  masonry,  then  grouted 
ill  place. 

All  bearings  for  plates  which  rotate  on  this  casting  are  to  be 
turned  smooth,  and  are  to  be  provided  with  suitable  oil- 
grooves,  so  they  can  be  easily  oiled. 


lOrf 


DE  roNtrnua. 


Spans  resting  on  druins  of  siuiill  diameter  in  pruportiou  to 
tlie  spun  lenglii  uie  to  bu  auchored  to  lliu  pivot-pier  by  nieans 
of  a  large  tinclior-rod  in  centre  of  pior,  extending  down  ten 
(10)  or  tifleen  (15)  feet  into  same.  This  rod  slitill  pass  throngli 
the  centre  canting  iind  tliroiigli  a  box  girder  over  tlie  centre  of 
the  drum,  whicli  girder  shull  rivet  into  eitiier  tiie  transverse  (»r 
the  longitudinal  girders.  Tlio  lower  end  of  the  rod  shall  p.-iss 
through  a  heavy  cast-iron  anchor-piece  embedded  in  the  con- 
crete of  the  pier.  Both  cutis  of  the  rod  shall  be  provided 
with  nuts  for  adjustment,  and  all  details  shall  be  made  strong 
enough  to  develop  the  full  strength  of  the  anchor-rod.  The 
upper  nut  shull  be  almost,  but  not  quite,  in  contact  wiih  a 
large  washcr-|>late  that  rests  on  the  box  girder.  The  si/.e  of 
the  anchor-rod  is  to  be  dotermineil  by  assuming  an  unbal- 
anced upward  wind  load  of  live  (5)  pounds  per  square  foot  on 
the  total  area  of  the  horizontal  projection  of  one  arm  of  the 
span. 

The  cap-plate  for  holding  down  the  top  connection-plate  for 
the  radial  struts  is  to  be  attached  to  the  toj)  of  the  centre  cast- 
ing by  means  of  a  bolt  tapped  into  same.  Tiii-t  bolt  is  to  be 
at  least  one  and  one-quarter  (1^)  inches  in  diameter. 

The  rack  for  turning  the  span  is  to  be  made  in  short  sec- 
tions, not  over  four  feet  long,  so  that  in  case  of  breakage  oidy 
a  small  portion  of  the  rack  need  be  replaced.  These  rack 
segments  are  to  be  bolted  to  the  lower  track  with  tap-bolts  not 
less  than  fifteen  sixteenths  (||)  of  an  inch  in  diameter,  and 
spaced  not  to  exceed  fifteen  (15)  inches  between  centres. 
Tliere  must  be  enough  of  them  in  any  case  in  any  one  sec- 
ment  of  the  track  to  resist,  with  a  good  margin  for  con'in- 
gencies,  the  entire  shear  (including  that  due  to  the  rotating 
moment)  caused  by  ih'  effort  of  the  pinion  or  pinions  that 
engage  with  said  segment.  The  letist  allowable  thickness  of 
metal  in  the  rack  shall  bo  one  and  one-eighth  (1^)  inches. 
The  ends  of  the  rack  segments  are  to  be  planed  so  as  to  secure 
close  contact,  and  the  abutting  ends  are  to  be  bolted  together 
with  turned  bolts  at  least  seven  eighths  (i)  of  an  inch  in 
diameter. 

The  bottom  of  the  rack  and  that  portion  of  lower  track 


SPKrlPlCATlONS    POU    HAILROAI)    DRAW-SPANS.    100 


upou  wbicli  I  lie  mck  boiirs  are  to  be  planed  smouth.  The 
width  of  the  hiise  of  the  rack  sliuU  beat  least  two  thirds  (})  of 
itH  height;  and  ribs  bracing  tlie  vertical  portion  to  the  base 
Khali  be  provided  at  distances  not  exceeding  eighteen  (18) 
inches. 

Dndnage-holes  not  less  than  ilirec  fourths  ())  of  an  inch  in 
diameter,  spaced  not  njore  tlian  two  (2)  feet  between  centres, 
shall  be  l)ored  in  tlie  lower-track  segments,  starling  just  back 
of  the  ra(;k  and  leading  to  the  outside  of  the  track. 

The  girders  (tver  the  drum  siiall  be  so  arranged  as  to  dis- 
tribulti  the  load  over  it  properly.  Tlie  number  of  bearing 
points  required  will  depend  upon  the  length  of  span,  the 
distance  from  centre  to  centre  of  trusses,  the  total  load  to  be 
carried,  and  the  economical  sizi!  of  pivot-pier.  The  arrange- 
ment of  the  supporting  girders  in  turn  depends  upon  the 
number  of  bearing  points  to  be  used  For  ordinary  single- 
track  bridges  up  to  three  hundred  (iJOO)  feel  in  length  a  very 
good  arrangement  of  girders  over  drum  is  secured  by  nniking 
the  diameter  of  the  drum  and  the  length  of  centre  panel  equal 
to  the  distance  from  (-entre  to  centre  of  trusses;  then  the  mid- 
dle points  of  both  tlie  longiliullnal  and  the  transverse  girders 
will  be  directly  over  the  web  of  the  drum,  thus  furnishing 
four  points  of  bearing.  Four  more  points  of  bearing  are 
secured  by  putting  in  short  diagonal  girders,  which  connect  to 
both  transverse  and  longitudinal  girders  and  bear  on  the  drum 
at  their  centres.  This  arrangement  gives  in  all  eight  (8)  points 
•  of  support. 

The  longitudinal,  transverse,  and  diagonal  girders  over  the 
drum  shall  be  .so  designed  that  'heir  rigidities  will  be  such  that 
when  deflected  under  llie  load  the  extreme  fibre-stress  will  be 
about  the  same  in  all  the  said  girders. 

The  bottom-chord  .stresses  iu  the  centre  panel  can  either  be 
carried  by  the  longitudinal  girders,  or  the  bottoni' chord  sec- 
tions can  be  continued  through  the  centre  panel,  the  longitu- 
dinal girders  being  placed  above  them,  and  steel  chairs  being 
inserteil  beneath  their  centres  to  furni.sh  bearings  on  the 
drum.  In  case  that  the  bottom-chord  stresses  are  carried  by 
the  longitudinal  girders,  ample  provision  must  be  made  for 


200 


DB  PONTIBUS. 


them,  as  well  as  for  the  bending  stresses,  in  designing  the 
sections  for  these  girders.  Wliere  the  clearance  over  tlie 
waterway  will  permit,  metal  can  be  saved  by  lett'ng  tbe  top 
flange  of  the  lougitudiual  girder  form  the  bottom  chord  of  the 
truss. 

In  any  arrangement  of  girders  over  the  drum,  bearing-plates 
at  least  one  (1)  inch  thick  must  be  used  between  the  top  flange 
of  the  drum  and  the  bottom  flanges  of  the  girders,  in  order 
to  make  the  points  of  concentration  well  defined,  and  so  as  to 
transmit  the  load  properly  from  girders  to  drum. 

All  girders  bearing  on  the  drum  are  to  have  stiffeners  on 
both  sides  of  their  webs  at  all  points  of  concentration;  and  in 
no  case  are  the  stiffeners  to  be  crimped,  but  are  to  have  fillers 
beneath.  They  must  have  close  bearings  at  top  and  bottom 
flanges,  and  are  to  be  proportioned  in  the  same  manner  as 
previously  specifitd  for  those  on  the  drum. 

The  rollers,  tracks,  drum,  and  girdeis  over  drum  shall  be 
completely  assembled  in  the  shop  before  shipment,  all  holes 
being  reamed  to  fit  and  the  sections  being  match-marked. 
Every  roller  must  have  a  true  bearing  on  both  the  upper  and 
the  lower  tracks  during  a  complete  revoluiiou  of  the  draw. 

Before  the  assembling  of  the  rollers  is  done  there  must  be 
nnirked  on  both  the  upper  and  the  lower  track  segments  a 
circle  of  the  sjime  diameter,  which  circles  will  come  a  trifle 
inside  of  the  exterior  ends  of  all  rollers;  then,  after  the  turn- 
table is  perfectly  adjusted,  each  roller  is  to  be  marked  where 
these  circles  touch  it.  After  the  turntable  is  disconnected 
each  roller  is  to  be  set  up  properly  in  a  lathe,  and  the  exterior 
periphery  is  to  be  chamfered  off  exactly  to  the  points  marked, 
so  that  •.i'hen  the  turntable  is  set  up  in  the  field,  if  the  exte- 
rior of  each  roller  is  brought  exactly  to  the  circles  on  the  two 
tracks,  the  rollers  will  all  be  in  their  proper  positions.  These 
lines  on  the  tracks  will  serve  also  afterwards  to  line  up  the 
rollers  whenever  the  turntable  is  to  be  adjusted. 


SPECIFICATIONS   FOR   hAlLHOAD   DRAW-SPAKS.   20l 


MACHINERY  FOR  TURNING  THE  SPAN  AND  LIPTING 
THE  ENDS  OF  S/.Mdi 

POWER. 

When  a  draw-span  is  to  be  opened  frequently,  some  kind  of 
mecimniciii  power  must  be  used.  The  kind  of  power  Ijol 
adupled  to  any  particular  span  depends  upon  a  number  of 
conditions,  more  especially  the  location  of  the  bridge. 

A  gasoline-engine  is  an  economic  and  convenient  form  of 
l)ower  for  small  spans  which  do  not  require  more  than  twelve 
(12)  or  fifteen  (1;"5)  horse-power  to  operate. 

Duplicate  electric  motors,  where  direct  connections  can  be 
mode  with  electric-liglit  or  street-railway  power-plants,  are 
very  efficient,  convenient,  and  reliable;  but  in  no  case  is  it 
safe  to  depend  upon  sturagebalteries  for  power.  Tlie  use  of 
electric  motive  power  is  therefore  confined  to  bridges  located 
in  or  near  towns  or  cities. 

Where  over  twelve  (12)  or  fifteen  (15)  horsepower  is  re- 
(juired  for  operating  the  spans,  and  where  electrical  conuic- 
tions  cannot  be  made,  the  steam-engine  is  the  best  form  of 
power  to  11:36,  except  i)ossibly  in  some  special  cases  where 
water-power  c;in  be  had  conveniently. 

Except  in  tjje  case  of  short,  light  drawbridges,  whenever 
met  hanical  power  is  employed  it  is  necessary  to  apply  the 
same  to  the  rack  by  two  pinions  located  diametrically  ojiposite 
each  other.  If  with  this  arrangement  tl»e  tooth  pressure  be 
still  too  high,  it  will  be  necessary  to  repbiee  each  jiinion  by  a 
l)air  of  pinions  located  as  close  together  as  practicable.  With 
pinions  located  far  apart  some  kind  of  an  equalizer  must  be 
enjployed  to  divide  the  work  equally  between  them,  on  ac- 
count of  the  unavoidable,  slight  irregularities  in  tlie  tooth- 
spacing  of  the  entire  rack.  When  electrical  power  is  adopted, 
the  equalizing  may  be  done  by  means  of  electrical  connections 
between  the  duplicate  motors;  but  with  any  other  power  a 
mechanical  equalizer  between  the  two  ladial  shafts  must  be 
employed.     There  will  be  no  equalizing  needed  between  tho 


P8%OVlNCi^i_    HDRARY 
VICTORIA,  B.  C. 


202 


DE  POKTIBUS 


two  piuioiis  of  each  pair,  on  account  of  their  being  piuctd  so 
close  logelher. 

With  the  eqiiftlizing  arrangement  just  specified,  it  is  legiti- 
mule  lo  iissunie  an  equal  division  of  work  among  all  the 
pillions  I  hat  engage  the  rack. 

No  matter  what  mechanical  power  be  used,  all  spans  must 
he  provided  also  with  hand-operating  machinery. 


le' 
on 


•METHOD  OP  DETERMININQ  POWER  REQUIRED  FOR 
OPERATING  THE  SPAN  AND  LIFTING  THE  ENDS. 


The  power  required  for  turning  any  span  is  to  be  deternuued 
by  the  following  formula. 


(1) 


H.P.  = 


.0125  IFp 
550 


where  W  ~  total  load  on  rollers  in  pounds,  and  «  =  velocity 
oil  pitch-circle  of  rack  in  feet  per  second.  The  value  of  v  is 
to  be  determined  by  the  formula 


V  ■— 


itD 
4^' 


where  D  =  diameter  of  pitcli-cirele  of  rack,  and  t  =  assumed 
time  in  seconds  for  luriiliig  tlie  draw  tiirough  one  fourth  (j)of 
a  revolution.  This  method  gives  the  power  required  under 
ordinary  conditions;  but  it  is  always  necessary  lo  figure  also 
the  power  required  to  open  the  span  against  an  assumed  un- 
balanced wiiul-piessiire.     This  is  to  be  delermined  as  follows: 

The  unbalanced  wind-pressure  on  one  arm  is  to  be  lak(!n  at 
five  (5)  pounds  per  square  foot  of  the  exposed  surface  of  the 
floor  and  both  trusses. 

Let  P  =  total  unbalanced  wind  load  on  one  arm  in  pounds, 
p.nd  V  =  velocity  of  travel  of  its  centre  of  pressure  iu  feet  per 
second;  then 


(8) 


H.P.  = 


650' 


SPECIFICATIOKS    Poll    UAlLROAl)    DIIAW-SPANS.   203 

The  value  of  v  is  to  be  deterniiued  by  assuiniug  iv  certain 
time  t,  in  seconds,  for  turning  the  draw  one  foiirtli  (})  of  a 
levolulion.  Let  I  =  distance  in  feet  of  tlie  centre  of  pressure 
on  one  arm  from  the  centre  of  tbe  drum;  then 


(8) 


Ttl 

2t' 


For  Mechanical-power  Turning  macJiinery  tbe  greatest  H.P. 
required  is  to  be  determined  us  follows: 

Case  1. — {(i)  By  Formula  (1)  determine  tbe  il.P.  required 
for  turning  tbe  span  in  the  least  time  in  which  it  is  probable 
thai  the  said  span  will  evjr  need  to  be  opened. 

Case  II.— {a)  By  Formula  (1)  determine  the  H.P.  required 
for  turning  tbe  span  in  twice  the  time  assumed  in  Case  I.  (b) 
By  Formula  (3)  determine  the  H.P.  required  for  ojieratiiig 
the  draw  against  tbe  unbalanced  wind  load  in  twice  tbe  time 
assumed  in  Case  I,  and  add  together  the  two  amounts  of  H.P. 
determined  by  (a)  and  (6).  Tbe  sum  will  be  the  greatest  H.P. 
required  for  Case  H. 

The  greatest  pressure  on  the  teetli  and  toision  on  shafts 
found  for  these  two  cases  is  to  be  used,  the  metal  being  strained 
on  tbe  extreme  libre  as  hereinafter  specified;  but  the  said  teetli 
and  shafting  must  also  be  figured  on  the  iissuinpiions  thai  the 
entire  availaljle  capacity  of  the  machinery  is  recpdred  ni<:rely 
lo  hold  the  draw  from  turning  under  an  exx'ssive  unbalanced 
wind-pressure,  and  that  under  these  conditions  tbe  metal  is 
strained  twice  as  high  as  hereinafter  specified. 

For  Hand  'luniliin  inachinery  the  H.P.  required  to  turn 
the  span  in  tbe  leant  time  in  which  it  is  probable  that  It  will 
ever  need  to  be  opened  by  man-power  is  to  be  found  b}'  the 
formula  previously  specified;  theu  tlie  number  of  men  re- 
(pnred  to  perform  this  work  is  to  be  deterniiiud  by  assuming 
that  six  (0)  men  are  equivalent  to  one  H.P.  In  proportioning 
all  parts  of  the  Jiand-operating  macihinery  there  slndl  be 
assumed  on  the  levers  as  many  men  as  are  required  l)y  the 
above  method,  each  man  exerting  a  horizontal  thrust  of  one 
hundred  and  twenty  (130)  pounds.    Under  such  conditions  the 


S04 


I)H    I'OXTIBUS. 


metal  is  to  be  strained  the  same  us  lioreiiiaftei'  specified  for 
macbinery  operaled  by  mechanical  power  uuder  ordinary  con- 
ditions. 

DETAILS  OF  MACHINERy. 


OPERATING   MACHtNERY. 

All  gear- wheels  are  to  be  of  cast  steel  with  cut  gears.  To 
(letenuine  the  size  of  any  gear-wheel,  the  tooth  pressure  on  the 
pitch-circle  is  first  to  be  found  as  follows : 

For  gears  moved  by  mechanical  power  only, 

where  II.  P.  ■=  horse-power  to  be  transmitted  by  gear,  v  =  ve- 
locity in  feet  per  second  at   its  pitch-circle,  and  P=:  tooth- 
pressure. 
For  gears  moved  by  hand-iwwer, 

P  =  120NM, 

where  N=  number  of  men,  M  =  multiple  of  lever  over  gear 
under  consideration,  and  P  —  tooth-pressure. 

Having  thus  determined  the  tooth- pressure,  the  pilch  can  be 
foimd  by  the  following  formida  ; 

p  =  .035  \'\P, 

for  gears  in  which  the  face  is  equal  to  2|  times  ti  e  pitch, 
where  p  =  pitch,  and  P—  total  tooth-pressure. 

Til  is  allows  an  extreme  fibre-stress  on  the  teeth  of  eight 
thousand  (8,000)  pounds  per  square  inch,  which  is  to  he  the 
standard  intensity  for  all  teeth  under  ordinary  conditions  of 
operation.  Beve'-gears  are  to  be  consiilercd  as  only  three 
fourths  (5)  as  strong  as  spur-gears  of  the  same  pitch  and  face. 
The  use  of  bevel-gears  with  very  thin  teeth  will  not  be  allowed, 
even  though  they  be  of  standard  pattern  ;  but  special  bevel- 
gears  with  thicker  teeth  than  usual  will  have  to  be  manufac- 
tured. 


SPECIFICATIONS    FOK   RAII.KOAI)    DRAW-SPANS.   J^'Oo 


All  gears  are  to  be  key-seated  ami  fliiished  in  accordance 
with  the  practice  of  llie  best  machine-sliops.  All  pinions 
gearing  into  the  rack  and  into  the  large  spiir-wiieels  are  to  be 
shrouded  on  top,  and  tlie  extra  sirengtii  obtained  by  this 
shrouding  is  not  to  be  counted  upon  in  pro[)ortioning  the  size 
of  the  teeth  of  the  pinion. 

All  shafting  is  to  be  of  cold-rolled  steel,  and  is  to  be  provided 
with  couplings,  collars,  and  keys  for  gears. 

All  couplings  must  be  strong  enougli  to  develop  the  full 
strength  of  the  shafting,  and  must  be  keyed  to  the  sumo, 
tl;inge-couplings  being  preferred.  All  couplings  are  to  be 
placed  as  near  the  bearings  as  praciicable. 

Suitable  collars  are  to  be  u.sed  wherever  they  are  necessary 
to  hold  the  shafting  from  moving  longitudinally. 

Tlie  greatest  allowable  length  .of  any  shaft  between  centres 
of  bearings  is  to  be  determiuel  by  the  formula 

z  =  75  v'f7^ 

where  L  —  the  iinstipported  length  in  inches,  and  d  =  diameter 
of  shaft  in  inches. 

The  diameter  reqtiired  for  any  siiaft  is  to  be  determined  by 
the  following  formula: 


=  W   IT' 


where  rf  =  diameter  required,  H.P  =  the  horse-power  to  be 
transmitted,  and  JV=the  number  of  revolutions  per  minute. 
Tins  will  allow  for  all  bending  lliat  will  come  on  any  well- 
designed  and  properly  supported  .'^hafl  under  ordiiuuy  con- 
ditions, and  provides  for  an  extreme  fibre-stress  of  about 
twelve  thousand  (13,000)  pounds  per  squaie  inch,  under  the 
assumption  that  the  twisting  moment  and  the  bending  moment 
are  about  equal. 

Every  shaft,  however,  after  being  designed  by  the  i)receding 
formula  must  be  checked  as  follows,  and  if  found  weak 
must  be  pro|ierly  strengthened  either  by  increasiii:;-  tiio 
difinu'tcr  or  by  reducing  the  lever  arm  or  arms  of  the  bending 
mommU 


200 


I)E    PONTIIJUS. 


First,  laud  the  twisting  inonifiit  iiiul  ihe  Ijoiiding  moment 
(including  lliat  ciuisi'd  by  the  weiglil  of  llie  sliiil't  ilticll)  by 
cnmpuling  tlie  toolb-piessure,  wbidi  is  tlie  foKx-  producing 
directly  these  moments,  culling  the  twis;ing  iuomh  nt  T  iind 
tlie  bending  moment  M.  The  eqiiivrdent  twisting  monunt 
for  !i  combination  of  tlies-e  two  moments  is  given  by  llic 
ec^uation 

T  ==  if  +  \/M-'  +  T\ 

where  7"  is  the  equivalent  twisting  moment. 

The  corresponding  extreme  tibre-stress  is  to  be  found  by  the 
equation 

2" 


/=5.1 


(/» 


wliere  d  is  the  diameter  of.  the  shaft,  and /is  the  extreme 
libie-slr(!'ss.  This  shoidd  never  exceed  twelve  thousand 
(12,000)  pounds  per  scpiaie  inch  for  all  ordinary  conditions  of 
operation,  or  twenty-four  thousand  (24,000)  pounds  per 
square  inch  for  the  unusual  conditions  of  the  machinery 
stalled  by  the  unbalanced  wind-pressure  when  working  at  its 
utmost  capacity. 

In  no  case  is  any  shaft  of  less  than  two  and  one-quarter  (2|) 
inches  in  diameter  to  be  used  for  any  part  of  the  machinery  of 
draw-spans. 

Siutable  cast-iron  boxes  are  to  be  provided  for  all  bearings. 
All  boxes,  bearings,  couplings,  collars,  etc.,  are  to  be  nuide  in 
accordance  with  the  best  machine-shop  practice.  Tiie  boxes 
for  the  liiie  of  shafting  running  to  ends  of  span  are  to  have 
wooden  shims  beneath  them  so  that  the  shaft  can  be  aligned 
perfectly  after  the  span  is  swung. 

The  hand-power  turidng-machinery  is  to  bo  so  arranged 
that  the  levers  cnn  be  conveniently  applied  to  slnifls  near  the 
centre  of  s]ian  for  bolli  the  turning  and  the  end-lifting 
machinery.  Shafts  must  also  be  provided  for  apjilying  the 
hand-power  levers  to  the  end-lifting  nuichincry  at  eadj  end 
of  the  span.  Suitable  hand-levers  arc  to  be  i)roviik'd  for  as 
many  men  as  are  required  for  operat  ng  the  draw.  These 
levers  are  to  be  constructed  entirely  of  steel,  ii.\cej)ting  only 


SPECIFICATIONS   FOR    RAILROAD    DRAW-SPANS.    207 


the  small  wooden  quarter-rounds  at  tlie  ends  by  which  the 
men  tuUc  hold. 

All  machinery  shall  be  so  arranged  that  the  span  can  bc 
turned  completely  around  in  cither  direction,  and  so  that  it  is 
reversible  in  every  partictdar. 

KND- 1.1  KTINO   AI'l'AUATUS. 

The  ends  aie  to  be  lifted  and  locket'  by  means  of  a  toggle 
mechanism  to  be  operated  by  screws  at  each  end  of  tlie  span. 
The  entire  machinery  is  to  be  made  strong  enough,  with  the 
previously  "^[lecilied  intensities  of  working-stresses,  to  exert  an 
upward  force  on  each  end  of  each  truss  equal  to  the  assumed 
uplift  in  case  of  mechanical  power  ;  or  to  transmit  to  the  end 
rollers  the  greatest  force  that  the  men  can  exert  on  the  hand 
levers,  assuming  that  iis  many  men  will  be  applied  then  to  as 
are  nqiiired  for  the  turning-machinery,  and  that  each  man 
exerts  a  horizontal  thrust  of  one  hundred  and  twenty  (120) 
pounds,  straining  the  metal  the  same  as  in  the  case  where 
tiie  power  is  mechatdcal. 

In  case  of  mechanical  power,  all  the  teeth  and  shafting  must 
also  be  ligured  on  the  assumption  that  the  en  lire  available 
capacity  of  the  machinery  is  required  merely  to  start  motion, 
and  that  under  this  condition  the  metal  is  strained  twice  as 
high  as  herein  specified. 

The  size  of  screw  required  is  to  be  determined  by  the 
following  formula : 

where  d  =  diameter  of  screw  at  base  of  threads,  and  P  = 
axial  pressure  on  screw. 

The  axial  pressure  is  to  be  determined  for  the  two  following 
cases,  the  greater  pressure  thus  found  being  adopted; 
Case  I.— 

_  2iih^ 
h    ' 

where  Ii=  total  assumed  upward  reaction  at  one  end  of  span, 
//  =  K'eatest  rise  of  ends  when  end  lifts  are  applied,  and 
h  =  travel  0/  iiut  on  screw  juecessarj  to  produce  the  rise  A'. 


208 


Di;    lOXTIBUS. 


The  factor  two  (2)  is  used  to  allow  one  hundred  (100)  per 
cent  for  fiiclion. 
Case  11. — 

P  =  80^i^, 

where  M  =  the  number  of  pounds  pressure  the  screw  will 
exert  for  one  pound  applied  on  the  lever,  and  N  =  number 
of  m.  II  on  said  lover.  By  using  eighty  (80)  instead  «)l'  one 
hundred  and  twenty  (120)  in  the  above  formula,  tliere  is  made 
an  allowance  of  tliirty-liiree  and  a  tinrd  ('i\i^)  per  cent  for 
friction,  which  is  certainly  lower  than  it  will  ever  be  under 
ordinary  wurLiug  conditions. 

Assundng  the  coelhcienl  of  friction  low  in  this  case  nnd<es 
an  error  on  the  side  of  safety. 

For  all  ordinary  conditions,  Case  II  will  give  the  greater 
value  for  P.  Tlie  threads  are  lo  be  standard  8(jiiare  thread-, 
and  the  nuts  which  work  on  liieiii  nve  to  be  made  long  enough 
to  keep  ihc  greatest  working  unit  pressure  on  said  threads 
down  to  five  hundred  (500)  pounds  per  scpiare  inch 

All  links  iised  in  the  toggle  mechanism  are  to  be  propor- 
tioned by  the  formula 

j>  =  10,000  -  ?^~, 
z 

where  I  =  greatest  ui\supporte(l  distance  between  tillers,  ex- 
cept in  links  in  which  only  one  filler  is  used  between  two  fiats, 
when  il  is  to  be  taken  as  the  entire  distance  from  centre  to 
centre  of  end- pins,  i  =  thickness  of  each  link,  and  p  =  the  in- 
tensity of  working  compressive  stress. 

In  no  case  is  the  diameter  of  any  pin  used  in  a  toggle  to  be 
less  than  two  and  a  half  (2^)  inches. 

Rail  lifts  arc  to  be  provided  in  connection  with  the  euil- 
lif;ing  toggle,  and  the  mechanism  therefor  is  to  be  so  designed 
tliat  the  rails  will  not  begin  to  rise  until  the  end  rolh'is  have 
been  drawn  from  their  bearings  on  the  end  shoes  The  rails 
shall  be  lifted  so  as  to  clear  by  one  (I)  inch  all  parts  over 
which  they  mu.st  jiass  in  ttirning,  under  the  assnmpiion  that 
the  temperature  of  the  top  chords  is  higher  by  tiurty  (30)  de- 
jprOes  Fahrenheit  than  that  of  the  bottom  chords. 


SPECIFICATIONS    FOR   UAILUOAU    DRAW-SI'AXS.    209 


aU- 


Suitable  guide-chairs  for  tKc  rails  near  the  cuds  of  tlio  span 
are  to  be  provided  beneath  the  same  ou  at  least  flfteeu  (15) 
ties  from  each  end  of  the  span.  These  chairs  must  be  either 
spiked  or  boUed  to  the  lies,  and  must  hold  tiie  rails  tirmly  in 
place.  Guide-rods  such  as  are  employed  in  ordinary  switcli- 
work  are  to  be  used  every  six  feet  between  the  portions  of  the 
rails  resting  in  the  guide  chairs. 

Tlie  strength  of  all  parts  of  the  rail-lifting  machinery  is  to 
be  determined  by  computing  the  force  necessary  to  deOect  the 
two  rails  the  required  amount  in  a  distance  of  twenty  {20) 
feet,  and  adding  tifty  (HO)  per  cent  thereto  for  friction. 

If  considered  necessary  for  any  particular  span,  latclies  are 
to  be  provided  for  holding  the  ends  in  place  ;  but  under  ordi- 
nary conditions  the  track-rails  and  the  end  rollers  are  all  that 
will  bti  recpiired. 

In  double-track  drawbridges  special  attention  must  be  piiid 
to  the  designing  of  not  ordy  the  lifting-gear,  but  also  the  trusses 
themselves,  in  order  to  ensure  that,  under  the  most  unfavor- 
able circumstances  possible,  there  shall  be  no  lifting  of  the 
ends  of  trusses  off  their  supports.  If  such  a  lifting  were  pos- 
sible, the  result  would  certainly  be  the  derailment  of  an  enter- 
ing train,  and  consequently  disaster  to  the  span.  To  prevent 
such  uplifting  the  trusses  must  be  deep  and  very  rigid,  and 
the  lift  of  the  ends  roust  be  from  one  (1)  to  two  (3)  inches,  ac- 
cording to  the  length  of  the  span. 


8HOUB  AND  END-BEAKINQ    UOLLEKS. 

Rollers  are  to  be  provided  beneath  the  end-pins  of  trusses 
and  attached  to  the  span  by  means  of  links  which  form  a  part 
of  the  toggle.  The  rollers  must  be  bored  so  aS  to  fit  ever  the 
pins  at  the  bottom  of  the  links.  Both  the  pins  and  the  inside 
of  the  rollers  mu'^t  be  finished  very  smooth;  and  provision 
must  be  made  for  oiling  I  he  bearings  between  them.  Tim 
allowable  intensity  for  bearing  between  rollers  and  pins  shall 
be  ten  thousand  (10,000)  pounds  per  square  inch  of  horizontal 
projection  of  pin  inside  of  the  roller. 

No  roller  shall  be  less  than  six  (6)  inches  in  diameter,  and 
the  pins  in:9{de  of  st\me  shall  not  be  less  than  three  and  seven- 


210 


DE   rONTIBUS. 


sixteenths  (J},''^)  iiicUes  net  in  diunieler.  Tliu  pluy  between 
rollers  and  their  pins  shall  not  be  over  one  tliirty-seeond  (j,'g) 
of  an  inch.  The  links  forming  tlic  support  for  the  ends  of 
trusses  are  to  be  proportioned  by  the  formula 


p  =  10,000  -  300 


where  p  =  intensity  of  working  compressive  stress,  I  = 
greatest  unsupported  length  of  one  link,  and  t  =  thickness  of 
same. 

In  all  drawbridges  where,  on  account  of  infrequent  opera- 
tion combined  with  great  changes  in  temperature  and  great 
length  of  arms,  there  is  a  tendency  to  drag  llie  rollers  longi- 
tudinally on  llieir  bearings,  the  detailing  of  the  link  siippoils 
must  be  sucii  as  to  provide  sulHcient  rigidily  to  overcome  the 
fricti(m  of  tiie  rollers  on  their  bearings,  and  thus  permit  the 
lifting  apparatus  to  accommodate  itself  to  extreme  changes  of 
temperature  without  overstraining  any  of  its  parts. 

Tbe  bearings  for  rollers  on  the  shoes  shall  be  cup|)ed  one- 
eighth  (|)  inch  or  more  in  depth  so  as  to  provide  ample 
bearing  area,  using  an  intensity  of  ten  tliou.^and  (10,000) 
poimds,  impact  being  included  in  the  caiculaled  load.  The 
shoes  to  receive  the  end  rollers  may  be  made  of  either  cast  or 
structural  steel,  and  are  to  be  anchored  firmly  to  the  masoiny 
The  two  shoes  at  one  end  of  span  are  to  be  connected  (o  each 
other  by  means  of  adjustable  rods  not  less  than  one  and  one- 
half  (li)  inches  in  diameter,  and  strong  enough  to  take  up  the 
entire  thrust  from  the  toggle. 

Shimmiug-plates  varying  in  thickness  from  one  fourth  (i) 
to  one  half  {\)  of  an  inch  and  of  a  total  depth  of  not  less  than 
three  (3)  inches  are  to  be  used  bencnth  the  shoes  so  as  to  pro- 
vide adjustment  for  the  ends  of  the  span. 

Shoiilders  must  be  provided  on  the  slioes  to  furnish  a  bear- 
ing for  the  rollers  when  they  are  lowured  l)y  tiie  toggle.  Each 
shoulder  must  be  turned  so  as  to  fit  the  roller  exactly,  when 
the  axis  of  the  pin  through  the  said  roller  is  in  the  vertical 
plane  of  the  truss.  The  height  of  these  siioulders  above  the 
bottom  of  the  rollers  shall  be  about  one  ihird  of  the  diameter 


SPECIFICATIONS   FOB   RAILROAD   DRAW  SPANS.   211 

of  mU\'  rollers,  but  never  enough  to  involve  Iho  jxtssibility  of 
collision  with  the  draw-spun  during  ils  revolutir)n  and  when 
tlie  lop  chords  thereof  are  thirty  {',]())  degrees  Fahrenheit 
wanner  limn  the  bottom  chords. 

All  purls  of  tiie  end-lifting  niuchiuery  must  be  finished  in 
ucconluiice  with  the  best  niachinc-shop  practice,  and  all 
sliding  surfaces  shall  be  provided  with  oil-holes  that  are  easily 
accessible. 

In  all  cases  end  tioor-beanis  with  double  webs  shall  be 
us-ed,  in  order  to  provide  proper  support  for  the  end-lifting 
machinery. 

Whenever  spans  are  to  be  floored  for  highway  traflic,  all 
koyliolcs  for  applying  hand-levers  are  to  be  provided  with 
suitable  cast-iron  caps. 

Whenever  practicable,  the  end-lifting  toggle  machinery  is 
to  be  assembled  in  the  shops  to  make  sure  that  it  will  work 
satisfactorily. 


HOUSES  AND  SUPPORTS. 

Wherever  mechanical  power  of  any  kind  is  to  be  used  for 
operating  any  draw-span,  a  suitable  house  is  to  be  provided 
for  same.  The  size  of  the  house  required  will  depend  upon 
the  kind  of  power  to  be  used,  and  the  amount  thereof.  All 
l)!irts  of  the  house  shall  be  durable  and  strong,  and  shall  be 
finished  in  a  first-class  and  workmanlike  manner.  A  sufHcieut 
number  of  windows  is  to  be  put  in  to  light  properly  all  parts 
of  the  building.  The  house  shall  be  placed  high  enough  in 
tlie  tower  to  give  the  required  clearance  beneath  its  supports, 
and,  where  shallow  trusses  are  used,  it  shall  be  placed  entirely 
al)ov(!  the  span.  The  supports  for  the  house  shall  be  designed 
to  curry  the  weiglil  of  the  latter  and  that  of  all  machinery  to 
b(!  placed  therein,  together  with  a  proper  allowance  for  live 
load.  In  general,  steel  beams  shall  be  used  for  llie  joists  sup- 
l)()rtiug  the  floor,  and  all  parts  of  the  latter  shall  be  made 
strong  enough  to  carry  three  hundred  and  fifty  (850)  pounds 
per  square  foo*. 

The  weight  of  the  house  and  its  machinery  must  always  be 


212 


I)l<;    I'ONTIHUS, 


considered  in  pioporlioning  all  parts  of  the  8tru(!lme  which 
will  he  iiffuctt'd  by  Iheso  loads,  whether  the  span  is  to  he  pro- 
vided with  niecluuiieal  power  at  first  or  not,  as  it  may  become 
necessary  later  on  to  put  it  in.  The  wind  load  on  the  house 
imist  also  he  considered  in  proportioning  the  tower  posts  and 
all  bracing  between  them. 


CAMBER  AND  DEPLECTION. 

The  lengths  of  all  truss  members  shall  be  such  that  when 
the  assumed  uplift  is  applied  at  the  ends  of  the  span,  and 
when  the  greatest  live  load  is  on  the  structure,  the  centre  lines 
of  the  bottom  chords  from  end  to  ind  of  span  will  lie  in  a 
horizontal  plane.  Tiie  vertical- movement  of  the  ends,  from 
the  condition  of  no  stress  in  the  chords,  vvlien  the  weight  of 
the  finished  span  is  supported  on  the  falsework,  to  the  condi 
tion  of  the  span  swung,  must  be  very  carefully  figured,  as 
upon  this  will  depend  the  camber  increments  or  decrements  in 
lengths  of  members,  the  cleaiauces,  adjusHnenls,  etc. 


CHAPTER  XVI. 

GENERAL  SPECIFICATIONS    GOVERNING    THE    DESIGNING  OF 
STEEL   HIGH  WAY   BRIDGES  AND   VIADUCTS. 

GENERAL  DESCRIPTION. 
CLASSIFICATION. 

Hroirw^AY  bridges  sball  be  divided  into  three  classes,  viz., 
Class  A,  wLicb  includes  those  that  are  subject  to  the  continued 
application  of  lieavy  loads  ;  Class  B,  which  includes  those 
tliat  lire  subject  to  the  occasional  application  of  heavy  loads  ; 
and  Class  i),  which  includes  those  for  ordinary,  light  traffic. 

In  general  it  may  be  stated  that  bridges  of  Class  A  are  for 
(li-nsely  populated  cities  ;  those  of  Class  B  for  smaller  cities 
and  manufacturing  districts  ;  and  those  of  Class  C  for  country 
roads. 


MATERIALS. 

All  parts  of  the  structure,  excepting  the  flooring  or  paving. 
ahnll,  for  all  spans  of  ordinary  lengths,  be  of  medium  steel, 
excepting  only  that  rivets  and  bolts  are  to  be  of  soft  steel,  and 
adjustable  members  of  either  soft  steel  or  wrought  iron.  For 
very  long  spans  high  steel  nuiy  be  used  for  top  chords,  in- 
dined  end  posts,  pins,  and  eye-bars  in  botloni  ciiords  and  in 
main  diagonals  of  panels  where  there  is  no  reversion  of  stress 
when  impact  is  included.  Cast  iron  will  not  be  allowed  to  be 
used  in  the  superstructure  of  any  highway  bridge  or  trestle, 
except  for  purely  ornamental  work,  cast  steel  being  employed 
wherever  important  castings  are  necessary. 

313 


214 


DE   PONTIBUS. 


JOTST8,  PLANKS,    GUAHD-TIMBEUS,  AND   WOODEN   IIANDKAILS. 

Joists,  plunks,  gutird-mi's,  haud-rails,  and  all  otbir  tiiiiber 
portious  of  the  structure  shall  be  of  long-leaf,  Southern,  yellow 
pine,  or  otlier  timber  which,  iu  the  opinion  of  the  Engineer,  is 
eqimiiy  good  and  serviceable. 

The  sizes  of  the  limber  joists  shall  be  sucli  as  to  give  the 
requisite  resistance  to  bending,  the  elTocl  of  impact  being  con- 
sidered ;  but  no  joibt  shall  be  less  than  three  (3)  inches  wide 
or  twelve  (12)  inches  deep. 

As  a  rule  the  depth  of  a  joist  shall  not  evceed  four  (4)  limes 
its  width.  Otherwise,  the  joists  shall  be  properly  bridged  at 
distances  not  exceeding  eiglit  (8)  feet. 

They  shall  be  proportioned  by  the  formula 

o 


wiiere  M  \h  the  greatest  bending  moment  in  inch-pounds  upon 
a  joist,  H  is  the  intensity  of  working-stress  in  pounds,  //  the 
width  of  the  joist  in  inches,  and  d  tlie  depth  of  same  in 
inches. 

Joists  shall  be  dapped  at  least  one-half  (^  inch  upon  their 
bearings,  and  shall  have  their  tops  brought  to  exact  level 
before  the  planks  are  laid  thereon. 

They  shall  be  spaced  not  to  exceed  two  (2)  feet  between 
centres;  shall,  preferably,  lap  by  each  other  so  as  to  extend 
over  tile  full  width  of  the  tloor-beam  ;  and  shall  be  separated 
half  an  inch,  so  as  to  permit  the  circulation  of  air.  The  out- 
side joists,  however,  shall  abut  so  as  to  provide;  flush  surfaces 
from  end  to  end  of  .span. 

Floorplaidis  for  the  main  roadway  shall  be  at  lensl  three  (:!) 
inches  thick  and  from  eight  (H)  to  ten  (10)  inches  wide,  and 
shall  be  laid  with  one-quarter  (|)  inch  openings.  Each  plank 
shall  be  spiked  to  er.ch  joist  on  which  it  rests  by  two  (2)  seven 
(7)  inch  cut  spikes,  the  holes  for  which  shall  be  bored  in  order 
to  avoid  splitting  the  timber,  or  else  by  two  (2)  seven  (7)  inch 
wire  uails. 


SFKCIFICATIONS  FOli  STEEL  HIGHWAY  BRIDGES.    215 


Whenever  n  wearing-floor  is  used,  the  lower  plunks  in\ist  be 
planed  on  the  upper  side  and  sized  to  a  uniform  thickness, 
and  the  wearing-floor  nuist  be  planed  on  the  lower  side  so  us 
to  ensure  a  perfect  bearing  between  upper  and  lower  floors. 

Floor-|)laidvs  for  fooiwnlks  shall  be  at  least  two  (2)  inches 
thick  and  not  nuich  more  or  less  than  six  (6)  inches  wide,  and 
shall  be  laid  wilh  one-half  (I)  inch  o|)enings.  Each  of  said 
plauks  shall  be  si)iked  to  each  joist  upon  which  it  rests  b}' 
two  (3)  six  (())  inch  cut  spikes,  the  hoUs  for  same  being  bored. 

All  planks  shall  be  laid  with  the  heart  side  down. 

Tliere  shall  be  a  wheel-guard  of  a  scantling  not  less  than 
four  (4)  inches  by  six  (6)  inciies  on  each  side  of  the  roadway  to 
prevent  wheel  hubs  from  striking  the  trusses.  It  is  to  lie  laid 
on  its  flat,  and  blocked  up  from  tlie  floor  by  shims  at  least  one 
(1)  foot  long,  six  (0)  inches  .vide,  and  two  (2)  inches  thick 
spaced  not  more  than  seven  (7)  feel  between  centres,  each 
sliim  lieing  sj)iked  to  the  floor  by  four  (4)  four  and-a-half  (4j) 
inch  cut  spikes.  The  guard  rails  are  to  be  bolted  lo  the  floor 
through  the  centre  of  each  shini  by  a  tlnci'Mpmrter  (J)  inch 
bolt,  which  nuist  also  i)ass  through  the  joist  beneath.  When 
the  guard-rails  are  bolted  to  the  wooden  hand-r.iil  posts,  the 
bolt-heads  are  to  I)e  countersunk  into  the  guard-rail,  so  as  to 
make  a  flush  surface  on  the  inner  face  of  same.  The  joints 
in  the  guardrail  are  to  be  lap-joints,  at  least  six  (0)  in(rhes 
long,  each  located  symmetrically  over  the  niiddli!  of  a  shim. 
When  a  bridge  is  on  a  heavy  grade,  the  inner,  upper  corners 
of  the  guard-rnils  are  lo  l)e  covered  with  steel  angles  fastened 
to  the  timber  by  counlersiudv  screws,  spaced  about  eighteen 
(18)  inches  apart,  so  as  to  protect  the  guard-rails  from  the 
injurious  eflfecls  of  using  them  instead  of  wheel-brakes  foi- 
heavily-loaded  wagons. 

Wln.n  wooden  hand-r.uls  are  employed,  they  are  to  be  nuule 
of  piiu!,  the  posts  being  4"  X  6"  X  4'  6''  to  5',  with  two  (2) 
runs  of  2"  X  0'  timbers— one  on  its  Hat  and  the  other  below 
on  edge  to  support  the  flrst  for  a  hauil-rail — and  one  (1)  run  of 
2"  X  12"  hub-plaidi. 

The  posts  are  to  be  spaced  not  to  exceed  ten  (10)  or,  prefer- 
ably, eight  (K)  feet  apart.     The  hand-railing  is  to  be  flrmly 


21(5 


DE  PONTIBUS. 


attached  to  the  bridge,  and  rigidly  binced.  When  tlie  rigidity 
of  a  band-railiug  is  dependent  upon  that  of  the  outer  joists, 
the  latter  must  be  properly  bridged  aud  stilt'ened.  Any  other 
wooden  haud-railiug  of  equal  strength  and  rigidity,  and  which 
is  satisfactory  to  the  Engineer,  will,  however,  be  accepted. 

When  iron  hand-railing  is  employed,  it  is  to  be  of  a  linn, 
substantial  pattern,  pleasing  to  the  eye,  and  rigidly  attached  to 
the  trusses  or  floor-beams.  Both  through  and  deck  bridg  jf  "o 
to  be  provided  with  a  hand-rail  on  each  side,  not  less  Uum 
three  and  a  half  (3^)  feet  high  above  the  floor.  lu  case  there 
be  any  liability  of  a  horse  jumping  over  this  railing,  its  height 
nuist  be  increased  to  four  and  a  half  (4^)  or  Ave  (5)  feet. 
There  must  be  a  hand-rail  on  the  outside  of  eao'.i  sidewalk, 
nut  less  than  three  and  a  half  (3})  feet  in  height  above  the 
floor. 


Fr.OOKINO  ON   APPUOACHES. 

All  floor-timbers,  guards,  and  railings  sliiiU  extend  over  all 
piers  and  abutments,  and  make  suitable  connection  with  the 
embankments  at  the  ends  of  the  8tru(;ture.  Aprons  or  rover- 
joints  of  steel  plate  shall  be  provided  at  the  ends  of  spans,  if 
required.  The  floors  of  the  sidewalks  shall  e.xtend  to  and  con- 
nect with  the  floor  of  the  main  roadway,  so  as  to  leave  no  open 
space  between  them. 


BTUKKT-nAHiUOAD    TKACKS. 

Should 'theie  be  one  or  more  street-railroad  tracks  crossing 
the  bridge,  ihere  must  be  directly  under  each  mil  a  joist  or 
stringer,  properly  proportioned  to  resist  the  effect  of  the  total 
maximum  load  on  the  rail  ;  and  the  bending  eilect  of  the 
concentrated  loads  upon  the  tloo;  beams  must  be  duly  con- 
sidered. 

The  rails  shall  be  So  laid  as  to  ofl"er  as  little  obstruction  as 
possible  to  the  wheels  of  vehi(;le.s. 


' 


PAVKD   FLOORS. 


Where  paved  floors  are  adopted,  tht  pavement  shall  be  of 
the  best  of  Its  kind,  and  shall  be  built  according  ti.  'he  latest 


SPECIPICATIOKS  FOR  STKKL  llfGHWAY  HUIDaES.    21^ 

and  most  approved  specifications.  Piived  Hoors  (ire  always  lo 
be  supported  by  steel  slringc^rs,  preferably  of  rolled  I  beams, 
spaced  generally  not  to  exceed  three  (3)  feet  six  (6)  inches  be- 
tween centres.  For  asphalt  or  stone-blook  pavements,  a 
buckleil-plate  tloor,  with  coucrele  thereon,  f>h:iil  be  used.  Tlie 
surface  of  the  pavement  must  be  thoroughly  drained  so  as  not 
to  retain  water,  and  the  upper  surface  of  the  buckled  plate, 
before  it  is  covered  with  the  concrete,  must  be  protected  from 
rusting  by  a  liberal  use  of  the  best  obluiiiable  preservative 
coating. 

When  wooden-block  paving  is  adopted,  it  may  rest  on  a 
timber  floor  from  four  (4)  to  five  (5)  inches  thick,  which  in 
turn  rests  on  and  is  spiked  to  timb'-r  shims  that  are  bolted  ef- 
fectively to  the  stejl  stringers. 

All  paved  floors  must  be  pitched  so  as  to  drain  transversely 
to  the  structure;  but  plank  floors  need  not  be  pitched,  as  the 
water  will  druiu  through  the  quaiter-iuch  openings. 


CLKAKANCKS. 

Tiie  smiillest  allowable  clrar  roadway  slmll  be  twenty  (20) 
feet,  measured  between  inclined  end  posts,  excepting  for  cheap 
(!ountry  bridges,  where  it  may  be  reduced  to  eighteen  (18) 
feet,  or  even  to  fourteen  (II)  IVct,  in  case  that  the  bridge  be 
so  short  that  no  piovision  need  be  made  for  teams  passing 
thereon. 

The  smallest  allowable  clear  headway  shall  be  fourteen  (14) 
feet,  except  for  bridges  in  (;ities  wliere  the  ordinances  require 
a  greater  height.  The  corner  brnckels  may,  however,  en- 
croach on  the  specified  clear  headway,  provided  they  do  not 
extend  either  laterally  or  downward  more  than  five  (5)  feet. 


KFFRrTIVK    I,KN«TI!S   AND   OBl'THS. 

See  Specifications  for  Railroad  Slnictures. 


BTVLES  OF  BHIDOKS  FOU   VAltlOUS  SPAN    LEN0TU8. 

In  general,  spans  of  and  below  twiiily  (20)  feet  are  to  con- 
sist  of  rolled   beams  or    simply  wooden    "jists;   spans  from 


218 


DK   PONTinUS. 


twenty  (20)  to  sixty  (60)  feet,  of  plate  gink-rs,  spans  from 
sixty  (60)  to  ninety  (90)  feet,  of  open- webbed,  riveted  girders  of 
single  cancellation,  or  pin-connected  "A"  trusses;  and  spans 
exctediug  ninety  (90)  feet,  of  pin-conuected  trusses. 

'  I'  :.«'  of  pony-truss  bridges  of  any  kind  is  prohibiiod,  ex- 
cept! ly  half-through,  plate  girder  spaos,  in  which  the 
lop  flaL^  i  are  held  rigidly  in  place  by  brackets  riveted  to 
cross-girders  that  are  spaced  generally  not  to  exceed  fifteen 
(15)  feet  apart. 

FORMS  OF  TKU88E8. 

The  forms  of  trusses  to  be  used  are  as  follows : 

For  pin-counecled  spans  up  to  ninety  (90)  feet,  the  "A" 
truss. 

For  open  webbed,  riveted  girders,  the  Warren  or  Triangu- 
lar g'nler,  with  verticals  dividing  the  piinels  ;  also  tiie  Pratt 
truss. 

For  deck-spans  carrying  joists  on  tlie  lop  chords,  liic 
Warren  or  Triangular  girder  with  verticals  dividing  tiie 
panels  of  the  top  chords. 

For  spans  between  ninety  i9())  feel  and  about  two  hundred 
and  fifty  (250)  feet,  Pratt  trusses  willi  top  cliords  either 
straight  or  polygonal. 

For  spans  exceeding  two  hundred  and  fifty  (250)  feel, 
Petit  trus.ses. 

It  is  undeistood  that  these  limiting  lengths  are  not  fixed 
absolutely,  as  the  best  limits  will  vary  somewhat  with  tiie 
width  of  bridge  and  the  live  load  to  be  carried. 


MAIN   MKMHKRB  OF   TKUH8  HUIDOEB. 

All  spans  of  every  kind  shall  hav(!  end  tloor-beams,  riveted 
rigidly  to  the  trusses  or  girders,  for  supporting  the  joists  or 
stringers. 

Steel  stringers  are,  preferably,  to  be  riveted  to  the  webs  of 
the  cross-girtlers,  but  wooden  joists  are  generally  to  rest  on 
top  of  the  latter. 

lu  geuer.il,  all  lru8.ses  shall  have  main  cud  p(^sts  inclined. 


SPPXIFICATIOJ^S  FOR  STEEL  HIGHWAY  imiDGES.    5il9 


All  trusses  shall  be  so  designed  ns  to  lulinit  of  uccurate 
calculiitious  of  all  stresses,  excepting  only  such  uuiniportunt 
cases  of  uiublguity  us  occur  whea  two  stiff  diagonals  are  used 
iu  a  middle  panel. 

lu  important  bridges  with  steel  stringers,  all  lateral  bracing 
and  other  sway-bracing  shall  be  rigid  above  and  below  ;  i.e. 
the  sections  must  be  capable  of  resisting  compression,  adjust- 
able rods  for  such  bracing  being  allowed  only  iu  towers  of 
draw-spans  and  in  the  lower  lateral  systems  of  deck  bridges  ; 
but,  in  cheap  country  bridges,  the  lateral  and  other  sway 
diagonals  may  be  adjustable  rods. 

Tiie  stiff  tliagonals  of  lower  lateral  systems,  wliich  shall  be 
of  double  cancellation,  shall  be  riveted  rigidly  to  all  the  steel 
stringers  where  they  cross  them. 

In  the  trusses  of  important  bridges  counterbracing  the  web 
shall  be  effected  by  using  stiff  diagonals,  hut  in  cheap  bridges 
it  may  be  done  by  using  counters  of  adjustable  rods. 

All  through-spans  shall  have  portal  bracing  at  each  end, 
carried  as  low  as  the  specified  clear  headroom  will  allow. 
The  portal  struts  shall  l)e  riveted  rigidly  to  tlie  web  or  both 
flanges  of  tiie  incline  1  cml  yjosts.  liiveling  portals  to  one 
flange  only  will  not  lie  allowed. 

Wlien  the  height  of  the  trusses  is  great  enough  to  permit, 
transverse,  vertical  sway-bracing  shall  be  employed  ;  other- 
wise, corner  brackets  of  proper  size,  strength,  and  rigidity 
are  to  be  riveted  between  the  posts  and  the  upper  lateral 
struts. 

Deck-bridges  shall,  as  a  matter  of  precixulion,  have  sway- 
diagonals  between  opposite  vertical  posts  of  sutticient  strength 
to  carry  one  half  of  a  panel-truss  live  load  with  its  impact 
allowance  ;  and  the  transverse  bracing  between  the  vertical 
or  inclined  posts  at  each  end  of  span  shall  be  sufliciently 
strong  to  transmit  properly  to  tiie  masonry  one  half  of  the 
total  wiud-prtssure  carried  by  tlie  upper  lateral  .system  of 
the  span. 

The  lower  lateral  systems  of  deck-bridges  may  be  made  of 
adjustable  rods  in  alternate  panels,  thus  leaving  every  other 
panel  unbraced,  and  forcing  the  wind-pressure  from  below  up 


320 


t)Vl   POXTIBU.'^. 


the  verticnl  braciug  ;iiid  to  the  ends  of  the  span  by  the  upper 
latenil  system. 

In  important  bridges,  suspenders  or  hip  verticals  and  two 
or  more  panel  leugtiis  of  bottom  chord  at  each  end  of  span 
shall,  preferably,  be  made  rigid  members,  except  that  eye-bars 
are  to  be  used  i  )r  bottom  chords  of  "  A  "  truss  bridges. 

Ail  Hoor-bean  s  are  to  be  riveted  to  the  truss-posts  in  truss- 
spans,  excepting  in  the  case  that  eye-ltars  be  used  for  suspend- 
ers or  hip  verticals.  In  such  cases  11  'or-beam  hangers  may  be 
used,  provided  Ihey  be  made  of  plates  or  shapes,  and  that  they 
be  stayed  at  their  upper  ends  against  all  possibility  of 
rotation. 

CONTINUOUS   SPANS. 

See  Siieciflcations  for  Railroad  Structures. 

TUESTLE- TOWERS. 

In  general,  the  descriptive  specifications  for  railroad 
trestles  are  to  ])e  followed  in  designing  highway  trestles  or 
viaducts,  except  that  in  cheap  s'ructures  all  sway-diagonals 
of  towers  may  be  made  of  adjustable  rods,  with  hori/ontiil 
struts  at  the  panel  points,  provided  that  the  struts  be  rigidly 
riveted  to  the  columns. 

CAMBER. 

All  trusses  must  be  provided  with  such  a  camber  that,  with 
the  heaviest  live  load  on  the  span,  the  total  camber  shall  never 
be  quite  taken  out  by  detlection.  With  parallel  chords, 
sufficient  camber  will  be  obtained  by  making  the  topclioid 
sections  longer  than  the  corresponding  bottom-chord  sections 
liy  five  thirty-seconds  (/j)  of  an  inch  for  each  ten  (10)  feet  of 
length. 

Phite  girders  and  shallow,  open-webbed,  riveted  girdeis 
should  m)t  be  given  any  camber. 


EXPANSION,    ANCHORAGE,    AND   NAME   PLATES. 

See  Specifications  for  Railroa<l  Structures. 


SPECIFICATIONS  FOR  STEEL  HIGHWAY  BlUDGliS.    ^;3l 


LOADS. 

The  loads  to  be  considered  in  designing  liighwiiy  l)ridg»!g 
and  trestles  me  the  followin  4'  ;  .-ind  ull  parts  of  sjunc  luu  to  be 
pi'oportioned  to  sustain  properl}' ihe  greatest  stresses  produced 
thereby  for  all  possible  coinlnniitions  of  the  various  loads,  ex- 
cepting only  that  the  live  load  and  wind  load  cannot  act  to- 
gether, unless  the  structure  carry  an  electric  railway;  for  the 
reason  that  no  person  would  venture  on  the  bridge  when  even 
one  half  of  the  assumed  wind-pressure  is  acting. 

A.  Live  Load. 

B.  Impact  Allowance  Load. 

C.  Dead  Load. 

D.  Direct  Wind  Load. 

E.  Indirect  Wind  Load  or  Transferred  Load. 

F.  Effects  of  Clmngcs  of  Temperature. 

When  a  highway  bridge  carries  an  electric  railway,  it  sh.-ill 
be  proportioned  also  for — 

G.  Traction  Load,  and, 
,  H.  Centrifugal  Load. 

In  calculating  tlie  stresses  caused  by  a  uniform  moving  load, 
the  load  shall  be  assumed  to  cover  the  panel  in  advance  of  the 
panel  point  considered;  but  the  half  panel  load  going  to  the 
forward  panel  point  will  be  ignored;  or,  in  other  words,  the 
•  uniform  load  will  be  treated  as  if  concentrated  at  the  various 
panel  points. 


LIVE   LOADS. 

The  uniformly  distributed  live  loads  per  square  foot  of  floor, 
including  the  entire  clear  widihs  of  both  main  roadway  and 
footwiilk.s,  shall  be  taken  from  the  curve  diagram  shown  on 
Plate  V. 

In  applying  these  curves  the  span  lengths  used  shall  be  as 
follows : 

For  stringers  and  joists,  a  single  panel  length  ;  for  floor- 
beams  and  single  panel  suspenders  with  their  coi responding 
primary  truss  struts,  two  (3)  panel  lengths;  for  hip  verticals 


233 


DE   rONTIBUS. 


of  Petit  trdsscs,  four  (4)  panel  lengths  ;  and  for  niuin-tniss 
members,  tlie  length  of  spun  loaded  when  tlie  member  under 
consideration  receives  its  muximum  stress. 

lu  the  case  of  bridges  with  exterior  sidewalks,  one  sidewalk 
oidy  and  the  roadway  arc  to  be  considered  loaded  wlien  pro- 
portioning the  beam-hangers  and  primary  truss  members  of 
all  bridge;s,  and  when  proportioidng  (he  main -truss  niembers 
of  all  spans  less  than  one  hundred  (100)  feet  for  bridges  of 
Class  A,  and  of  all  spans  less  than  eighty  C80)  feel  for  bridges 
of  Classes  U  and  C.  In  all  other  cases  both  of  I  he  sidewalks 
and  the  roadway  are  to  be  considered  loaded.  Tlie  eccentric 
loading  increases  (he  live  load  per  truss.  But,  wlien  a  bridge 
has  only  one  exterior  sidewalk,  tiie  effect  of  the  eccentric 
loading  is  to  be  considered  to  act  upon  the  whole  of  the  nearer 
truss,  and  the  sidewalk  is  to  be  considered  empty  wlien  cal- 
culating the  stresses  in  the  fartlier  truss.  Floor-beams  of 
bridges  with  one  or  two  exterior  sidewalks  are  to  be  propor- 
tioned on  the  as.sumption  that,  first,  the  main  roadway  is 
loaded,  and  the  sidewalk  or  sidewalksare  empty;  and,  second, 
that  the  main  roadway  is  empty,  and  the  sidewalk  or  sidewalks 
are  h)aded,  due  account  being  taken  of  tlie  effect  of  reversing 
stresses  as  hereafter  specified. 

In  addition  to  the  preceding  loads,  the  floor,  joists,  floor- 
beams,  beam-hangers,  and  primary-truss  members  are  to  be 
proportioned  for  the  following  concentrated  loads,  which  are, 
however,  supposed  to  occupy  a  whole  panel  length  of  the  main' 
roadway  to  the  exclusion  of  the  other  live  loads  there  (except- 
ing only  the  electric-railway  live  load). 


CLASS  A. 

A  road-roller  weighing  thirty  thousand  (30,000)  pounds,  of 
which  twelve  thousand  (13,000)  jumnds  are  concentrated  upon 
the  roller  in  front  of  the  machine,  and  nine  thousand  (9000) 
pounds  on  each  of  the  wheels  at  the  rear,  the  distance  between 
the  central  planes  of  these  wheels  being  five  (5)  feet,  and  that 
between  their  axis  and  the  axis  of  the  front  roller  eleven  (11) 
feet.  The  width  of  the  front  roller  is  to  be  four  (4)  feet,  and 
that  of  each  nar  wheel  one  foot  eight  inches  (1'  8"). 


Sl'KClFlCATIONb  FOR  STKKL  HIGHWAY  URIDUKS.    X*5i3 


CLASS  B. 

A  concentrated  loiul  of  sixtem  iliousjnKi  (l(»,00())  pounds 
equally  distriliiited  upon  two  pairs  of  wIiclI.s,  tiie  axes  of 
wliich  are  eight  (n)  fe(;l  npurl,  and  the  central  pluues  of  the 
wheels  six  (6)  feet  apart. 

CLASS  c. 

A  concentrated  load  of  ten  thousand  ( 10,000)  pounds  dis- 
tributed in  tlic  same  manner  as  for  Class  B. 

The  road-roller  load  is  assumed  to  be  eciually  tlivided  lie- 
tween  all  of  the  joists  that  it  can  cover,  and  the  wheel  loads 
for  Classes  B  and  C  equally  between  two  joists. 

In  case  that  the  highway  bridge  or  trestle  carries  a  single- 
track  line  of  electric  road,  that  one  of  the  four  car  or  train 
lo'uls,  shown  on  Plate  VI,  which  most  closely  approximates 
lo  the  greatest  railway  load  that  will  be  carried  by  the 
structure  is  to  be  adopted,  and  is  to  be  assumed  to  occupy 
ten  (10)  feet  in  width  of  the  entire  clear  roadway  of  the  span 
to  ihe  exclusion  of  all  other  live  loads  on  stud  ten  (10)  feet, 
except  as  hereinafter  specified  for  fioor-beams  and  primary- 
truss  members. 

The  equivalent  luiiformly  distributed  live  loads,  given  by 
the  curves  on  Plate  VI,  are  to  be  used  when  making  compu- 
talioni  instead  of  the  concentrations  just  specified. 

The  impact  allowance  for  these  electric  railway  loads  is  to 
be  taken  from  the  Specifications  for  Railroad  Structures. 

The  t\(K>v  system  and  primary-truss  members  are  to  be 
figured  for  these  electric  train  loads  when  passing  either  the 
road-roller  or  the  heavy  wagon-load;  and  the  trusses  as  a 
whole  are  to  be  figured  for  a  uniform  load  found  by  combin- 
ing the  equivalent  electric  load,  considering  it  to  occu|>y  ten 
(10)  feet  of  roadway,  together  with  its  impact  allowance,  with 
the  regular  uiuform  live  load  per  square  foot  of  floor  on  the 
remaining  width  of  ciear  roadway,  together  with  its  proper 
1  r.pacl  allowance,  provided  that  the  equivalent  live  load  per 
lineal  foot  for  the  cars,  plus  the  proper  impact  allowance,  ex- 
ceetl  %he  regular  live  load  for  a  ten  (10)  foot  width  of  roadway, 


324 


|)K    I'ONTIP.US. 


phis  its  proper  impact  iilluwancc.     If  it  do  not  so  exceed,  the 
rcgidiir  uniform  live  loud  is  to  be  employed. 

IMl'ACTAIiLOWANCE   LOAD 

The  impact-Hllowaiice  load  is  to  be  a  percentage  of  the  uni- 
form  live  load,  found  by  the  formtda 


P  = 


10000 
/.  +  150' 


where  P  is  the  percentage  and  L  the  length  in  feet  of  spau  or 
portion  of  span  that  is  covered  by  the  live  loud,  vvlien  the 
member  cjnbidered  is  subjected  to  its  maximum  stress. 

DEAD  LOAD. 

See  Specifications  for  Railroad  Structure. 

WIND    LOADS. 

For  highway  structures  the  wind  loads  per  lineal  foot  of 
span  for  both  the  lojided  and  the  unloaded  clio.ds  are  to  be 
taken  from  the  curves  shown  on  Plate  VIII. 

This  diagram  was  figured  for  a  clear  roadway  of  twenty  (30) 
feet.  For  wider  structures,  the  wind  l')ads  are  to  be  iucrea.sed 
two  (2)  i^er  cent  for  each  foot  of  width  in  excess  of  twenty 
(20). 

The  wind  loads  given  on  the  diagram  have  been  computed 
from  detailed  designs  for  simple  spans  up  to  seven  liundred 
and  fifty  (750)  feel  in  length,  but  beyond  this  limit  they  have 
been  assumed;  consequently,  in  designing  spans  of  greater 
length  than  this,  it  will  be  necessary  to  check  tlie  assumed 
wind-pressure  after  the  sections  are  proportioned,  using  an  in- 
tensity of  twenty-five  (3."))  pounds  per  square  foot. 

The  intensities  employed  in  preparing  the  curves  varied  from 
forty  (40)  pounds  for  very  short  spans  to  twenty -five  (25) 
pounds  for  very  long  ones. 

For  viaducts,  the  wind  pressure  on  the  empty  strmture  is 
to  be  assumed  as  three  hundred  (300)  pounds  per  linea'  foot 
on  the  spans  at  the  level  of  the  floor,  and  two  hundred  and 
fifty  (250)  pounds  for  each  vertical  foot  of  each  entire  tower, 


SPECIFICA1I0.S>  FOlt  HTKKI-  lIKillWAY   HKIDfJKS.    32.5 

The  wind  loads  for  longitudinal  brnciiii!:  me  to  bo  taken  us 
seven  tenths  (0.7)  of  those  for  the  transverse  bracing. 

For  vialucts  carrying  electric  trains,  tiie  wind  loads  are  to 
be  taken  from  the  Specifications  for  Railroad  Sinictures. 

All  wind  loads  are  to  be  treated  as  inomng  'ouds. 

INDIIIECT   WIND   LOAD   OR  TltANHFKIlRED   I-OAD. 

See  Specifications  for  Railroad  Structures. 

TKACTION    liOAD. 

See  Specifications  for  Railroad  Structures. 

CKNTIJIFUGAL   liOAD. 

See  Specifications  for  Railroad  Structures. 

EFFECTS  OF  CHAN(}EB  OF  TEMPEKATURE. 

See  Specifications  for  Railroad  Structures. 


INTENSITIES  OF  W^OBKING-STRESSES. 
See  Specifications  for  Railroad  Structures. 

HEARINGS  UPON   MASONRY. 

See  Specifications  for  Railroad  Structures. 

REVER8ING-8TUES8ES. 

See  Specifications  for  Railroad  Structures. 

NET   SECTION. 

See  Specifications  for  Railroad  Structures. 

BENDING   MOMENTS  ON   PINS. 

See  Specifications  for  Railroad  Structures. 

COMBINATIONS  OF   STRESSES. 

The  Specifications  for  Railroad  Structures  under  this  head 
ing  are  to  be  followed,  with  this  exception  :  in  bridges  and 
viaducts  that  do  not  carry  trains,  the  live  load  and  the  wincj 
load  are  assumed  not  to  act  simultaneously. 


226 


|)K    I'ONTIIHS, 


BKNDINO    »)N   Tor   CHOUDH. 

See  Specirtcatioiis  for  Railrojul  Striu;tiire.s. 

UGNDINO   ON    INCLINKD  ENO   I'OHTH. 

The  Speciti(;HliouH  for  Kitilroiul  Blnictiires  under  tliift  head- 
ing lire  to  be  follo'wed,  witli  tliis  exception:  in  bridges  that  do 
not  carry  trains,  the  live  iimil  and  the  wind  load  are  assumed 
nut  to  act  simultaneously. 

BENDING   DUE   TO  WEIOHT  OK   MEMUEK. 

yee  iSpeeificalions  for  Railroad  Structures. 

OENEUAIi   LIMITS    IN    DKHIONINO. 

The  folliiwing  geiural  limits  shall  be  adhered  to  in  design- 
ing highway  bridges  and  viaducts  : 

The  perpendicular  distance  between  central  planes  of  trusses 
shall  never  be  less  than  one  twentieth  (j'^)  of  the  span. 

The  length  of  any  bracket  cantilevered  beyond  a  truss  or 
girder  shall  never  exceed  one  Iialf  of  the  perpendicular  distance 
between  the  central  planes  of  adjacent  tiusses  or  girders,  unless 
there  be  more  than  two  trusses  to  the  span. 

No  metal  less  than  five  sixteenths  (/g)  of  an  inch  in  thick- 
ness shall  be  used,  except  for  filling-plates;  and  in  important 
bridges  this  limit  shall  be  increased  to  three  eighths  (g)  of  itn 
inch. 

The  least  allowable  thicknesses  of  webs  of  rolled  I  beams 
shall  be  as  follows: 

24"  I  beams V*«"  webs. 

20  "      "     J 

18   "      "     {g 

15   "      "     f 

12   "      "     }g 

No  channel  less  than  six  (G)  inches  in  depth  shall  be  used 
exc  pt  for  lateral  struts,  in  which  five  (5)  inch  channels  may  be 
employed. 

No  angles  less  than  2|"  X  2^"  X  rs"  shall  be  used  except 
for  lacing, 


SI'ECIKICATION.S  FUlt  STKKL  IIKHIWAY   HRIDCSES.     127 

No  cye-bui8  icHH  ihuii  thrvc  (3)  iuclieudcep  or  fiveuiglilhs  (I) 
of  Ml  iiH'lt  thick  slitill  be  uinplo^ed;  uiid  the  depths  of  eye-bars 
for  chords  and  niaiu  diugoimis  shall  uot  be  less  than  one 
sixtieth  (^'g)  of  the  lioii/.oittal  length  o(  same. 

No  Hiijustablv  rod  shall  have  loss  than  three  quarters  (J)  of 
a  stiuaru  iu(;ii  ul  cruss-section. 

The  shoilest  span  Icn^ih  for  trusses  with  polygonal  lop 
chords  shiill  be  one  hundred  iind  sixty  (IGO)  feet. 

The  limit  of  span  length  in  wiiich  stct'l  stringers  can  be 
riveted  continuously  from  end  to  end  o'  span  shall  be  two 
hundred  (200)  feet.  Beyond  thin  Hunt  slidiugbearings  must 
be  used  at  one  or  more  intermediate  panel  points;  and  in  no 
span  shall  there  be  a  length  of  coniinuously  riveted  stringers 
exceeding  two  hundred  (200)  feet. 

For  all  compression- members  of  trusses  and  for  columns  of 
viaducts  the  greatest  ratio  of  unsupported  length  to  least  radius 
of  gyration  shall  be  one  hundred  and  twenty  (120).  excepting 
those  members  whos»^  main  function  is  to  resist  tension.  In 
these  the  limit  may  be  raised  to  one  hundred  and  fifty  (150). 

The  corresponding  limit  for  all  struts  belonging  to  sway- 
bracing  shall  ale')  be  one  hundred  and  fifty  (150). 

OENERAIi  PBINCIl'LES  IN  DESIQlJING  ALL  HIGH- 
WAY STBUCTUREt>. 

See  Specification  for  Uinlroad  Structures. 

RIVETING. 

In  general,  the  specifications  for  rivt  ting  given  for  railroad 
structures  shall  apply  also  to  highway  structures,  except  that 
in  the  latter  tlte  diameters  for  rivets  may  be  reduced  to  three 
quarters  (J)  of  an  inch  for  ordinary  work. 


PETAILS  OP  DESIGN  FOR   ROLLED  I-BEAM  SPANS. 

Rolled  I  beams  used  as  longitudinal  girdi-ns  shall  have, 
preferably,  a  depth  not  less  than  one  fifteeulh  (^b)  of  the  span. 
Tlicy  shall  be  proportioned  by  their  moments  of  inertia.  The 
spacing  shall  generally  not  excee(|  three  (3)  feet  six  (6)  inches. 


S'SH 


DK    PONTIHUS. 


Provided  Unit  woe  den  shims  be  boiled  to  the  top  tluiigcs  for 
spiking  the  plunks  iliereto,  no  swuj'-bracing  will  be  retjuircd  ; 
but  otherwise  it  must  be  ustd.  Eucli  I  beam  is  to  )>iive  at  eacii 
end  n  pair  of  stiffening  angles,  titling  tigiitly  at  both  top  and 
bottom  to  the  tlanges,  to  carry  the  load  lo  the  nnisomy  and  to 
form  part  of  the  end  braciug-friiines.  Each  i)air  of  girders  is 
lo  have  a  bracing-frame  at  each  end;  and  under  each  end  of 
each  1  beam  there  is  to  be  liveti d  a  l)earing-plate  of  proper 
area  and  thickness  (never  Kss  than  live  eighths  [^]  in(;li)  t.» 
distribute  the  load  unifonnly  over  the  masonry,  8ai<l  plate 
being  bolted  effectively  to  the  latter,  witli  due  provision  for 
expansion  and  contraction. 

DETAILS  OP  DESIGN  FOR  PLATB-OIRDER  SPANS. 

In  designing  plate-girder  sjians  for  highway  structures,  the 
corresponding  specifications  ior  railroad  structures  are  to  be 
followed,  except  that  tlie  depths  of  girders  shall  preferably  be 
not  less  than  one  twelfth  (j'a)  of  (heir  span,  that  nu'tal  tivc 
sixteenths  (f^)  inch  tliick  may  be  used,  and  that  the  stiffening 
angles  may  be  made  as  snii'.ll  as  two  and  ;;,  half  (2^)  by  two 
and  a  half  (2i)  inches. 

DI  TAILS  OP  DESIGN  POR  OPEN-WEBBED,  RIVETED 
GIRDER  SPANS. 

See  Specifications  for  llailroiid  Structures. 


DETAILS  OP  DESIGN  POB  PIN-CONNECTED  SPANS. 

The  sections  of  lop  chords  and  inclined  end  posts  of  ihrotigh- 
spaus  shall  consist,  generally,  of  two  rolled  or  built  channels 
and  a  single  cover-plate.  In  the  case  of  built  channels,  the 
section  of  the  member  must  be  so  proportioned  as  to  bring  iis 
centre  of  gravity  as  near  .-xs  possible  to  tlie  middle  of  the 
webs. 

Main  vertical  posts  shall  generally  !«'  composed  of  two 
laced  channels,  preferably  rolled  ones,  iilthough  bulk  chan- 
nels may  be  used  where  large  sections  are  required. 


sPKr!iKi(;ATroNs  for  steel  highway  imrDGES.  220 


Secoiidiiry  vertical  posts  may  be  made  of  iwo  rolled  cljaii- 
uel-j  laced,  or  of  four  angles  in  the  form  of  an  1  with  a  single 
lino  of  lacing.  These  secoiulury  vertical  posts  should  be 
riveted  to  the  top  chords  instead  of  being  plu-counected 
thereto,  as  in  the  case  of  the  main  vertical  posts. 

Tlie  chnimels  of  vertical  posts  may  have  their  flanges  turned 
either  inward  or  outward,  as  desired,  or  so  as  best  to  suit  the 
general  detailing  of  the  truss. 

Stiff  bottom  chords  and  inclined  web-struts  may  be  made  of 
either  two  channels  wiih  two  lines  of  lacing  or  of  four  angles 
with  one  line  of  lacing,  the  use  of  trussed  eye-bars  for  stmts 
l)eing  prohibited. 

Upper  lateral  struts,  overhead  transverse  struts,  and  web- 
stiffening  .struts  shall  preferably  be  made  of  four  angles  wilh 
one  line  of  lacing.  In  cjise,  however,  tlie  said  angles  be 
spaced  very  far  apart,  as  in  lateral  struts  connecting  deep  top 
chords,  they  are  to  be  plaoeil  on  the  corners  of  a  rectangle, 
v. ith  their  legs  turned  inward,  and  laced  on  all  four  faces  r)f 
the  box  strut  thus  fornu'd. 

Eye-bars  are  to  be  used  for  all  bottom  chords  and  main 
diagonals  that  do  not  require  to  l>e  stiffened. 

CounkTs,  wium  employed,  can  be  of  either  rounds,  squares, 
or  tliils.  Tliest!  mid  all  other  adjustable  nu-nibcrs  are  lo  lia\e 
their  ends  enlarged  for  tlie  screw-threads  (unless  soft  steel, 
cold-pressed  threads  be  nsed),  so  that  the  diameter  at  the  bot- 
tom of  the  thread  shall  bo  one  eighth  {\]  >f  an  inch  gr.aler 
than  tiiat  of  the  body  of  a  rour.d  rod  of  area  eqnal  to  tliat 
of  the  adjustable  piece. 

Diagoinils  for  upper  lateral  systems  and  vertical  sway- 
bracing  siiall,  preferably,  be  l)uilt  of  foiw  angles  in  the  form 
of  an  I,  with  a  single  line  of  lacing;  but  for  structures  where 
this  section  would  involve  an  extiavagani  use  of  metal,  two  of 
tlie  angles,  one  at  tlie  top  and  one  at  the  bottom,  may  be 
onutted,  thus  making  each  strut  consist  of  two  angles  laced, 
provided,  of  conrse,  th;it  where  the  struts  cross  they  shall  lie 
rigidly  connected  by  two  plates  of  ample  size.  This  unbal- 
anced section  for  such  diagonals  is  to  be  avo'ded  whenever  it 
can  be  done  without  undue  use  of  nu^al.     Ii.   ,o  case,  though, 


230 


DE   PONTIBtJS. 


will  it  l)e  permissible  to  use  angles  in  tension  that  ure  not 
capable  of  resisting  properly  Uie  possible  compressive  stresses, 
with  due  regard  for  the  specitied  limit  of  ratio  of  uiisup- 
polled  length  to  least  rn.lius  of  gyration. 

In  cheap  highway  bridges  the  lateral  diagonals  may  be  made 
of  adjustable  reds  with  right  and  left  clevises  at  their  ends, 
by  wiiich  they  are  to  be  connected  through  pins  to  corner- 
plates  that  are  riveted  to  both  the  lateral  ijtrut  and  the  truss 
member.  The  ordinary  detail  consisting  of  two  or  three  short 
pieces  of  angio  riveted  on  top  of  the  cover-plate,  and  between 
two  of  which  the  rod  lies,  will  not  be  permitted.  Where 
adjustable  rods  are  employed,  the  struts  to  the  ends  of  which 
they  attach  must  be  figure  1  for  a  total  compressive  stress  e(|ual 
to  the  suiu  of  the  components  (in  the  direction  of  said  strut)  of 
the  greatest  allowable  working-stresses  on  all  of  the  adjust- 
able rods  meeliug  at  one  eiul  of  said  strut.  While  this 
method  gives  an  excessive  stress  for  llie  strut,  I  lie  etfut  will 
be  a  desirable  error  on  the  side  of  safety  and  riiiidity. 

In  designing  transverse  lateral  and  overhead  struts  and  their 
connections,  it  must  be  remembered  that  their  main  function 
is  to  hold  rigidly  the  chords  or  posts  to  jdace  and  line,  and  not 
merely  to  resist  as  columns  the  greatest  calculated  direct 
stresses  to  which  they  nniy  be  subjected.  For  this  reason 
such  st'^uts  should  have  ample  section  for  righlity,  and  the 
conrcctiug  plates  at  their  ends  should  grip  both  connected 
m«  mbers  effectively. 

Where  built  stringeis  are  used  for  the  floor  sysleui,  they 
shall  be  made  without  cover-plates,  and  generally  of  the 
economic  depth  in  respect  to  total  weight  of  metal,  hut  never 
less  in  depth  than  one  flfleeuth  {j\)  of  the  span.  No  splices 
w!ll  be  allowed  in  their  flanges  nor  any  in  their  webs,  pro- 
viiled  that  sutticiently  long  web-plates  are  procurable.  The 
compressioiitlanges  shall  be  made  of  the  same  gross  section 
as  the  tension-tlanges,  and  they  shall  be  so  stiffened  that  the 
tinsupported  length  shall  never  exceed  sixteen  (10)  times  the 
width  of  tiauge.  Rigid  diagonal  bracing  of  angles  is  to  be 
used  between  the  top  tlanges  oC  such  stringer-,  luiless  they  be 
held  rigidly  in  place  by  the  dooring;  and  rigid  bracing-frames 


V 
r 

n 
ti 


sPK(;iprcArroNrs  forstkml  Hr(ji£\VAY  hhidoes.  2;]l 


TllL- 


are  to  be  oniplryed  between  tbe  ends  of  adjiKent  stringers  at 
all  expansion  points.  Wliere  sucli  stringers  are  used,  tbe 
lower  lateral  system  must  invariably  consist  of  rigid  sectiotis, 
eaeh  piece  being  riveted  to  eiicli  stringer  wbere  it  crosses  tbe 
same. 

Ill  respect  t'>  stiffening  angles  for  siringers,  tbe  rules  govern- 
ing Ibose  for  pl.ile-girdcr  spans  are  to  be  followed  ;  but  tbe 
end  slill'eners  are  to  be  faced  or  otherwise  treated  so  as  to 
make  the  siringers  of  exact  length  liiroughout,  and  so  as  to 
effect  a  uniform  bearing  of  tbe  end  slilTeners  against  tbe  webs 
of  the  cross-girders. 

In  respect  to  tlie  proportioning  of  tlangcs  and  number  of 
rivets  required,  llie  rules  given  for  plate-girder  spans  are  to 
apply  also  to  stringcns.  The  said  rules  are  to  apply  to  cross- 
girders,  as  shall  also  those  relating  to  siitfeners,  splices,  cover- 
plate^i,  and  size  of  eoiupressiou-rtauges,  that  are  given  for 
plate-girder  spans.  Wiierever  it  is  i'  saiy  to  notch  out  the 
corners  of  the  cross-girders  to  clea  Iiords,  the  greatest 

cure  must  be  taken  to  provide  an  adequaie  means  for  trans 
ferring  tbe  shear  to  the  posts  without  impair! t;  ojther  tbe 
strength  or  the  rigidity.  If  necessary,  in  through-bridges,  the 
web  of  the  cross-girder  may  be  divided  into  three  parts  so  as 
to  let  the  end  portions  project  above  tbe  top  fl mge  and  form 
brackets  that  will  afford  opportunity  for  using  an  ample  iiun;- 
ber  of  rivets  to  connect  to  the  posts,  and  will  strengthen  pr^  *- 
erly  tbe  otherwise  weakened  cross-girder. 

All  plates,  angles,  and  cliannels  used  in  built  members  of 
trusses  must,  if  practicable,  be  ordered  the  full  length  of  tlie 
member  ;  otherwise  the  splices  must  develop  the  full  strengii 
of  the  niemb(;r  without  any  reliance  being  placed  on  tbe  abut- 
ting ends  for  carrying  compression. 

But  in  total  splices  at  the  ends  of  sections  perfect  abutting 
of  the  dressed  ends  is  to  be  relied  upon.  However,  the  splice- 
plates  even  there  must  be  of  ample  size  and  strength  for  both 
rigidity  and  continuity. 

The  unsupported  width  of  plates  strained  in  compression, 
measuring  between  centre  lines  of  rivets,  shall  not  exceed 
tliirty-two  (32)  times  their  thickness,  except   in  the  case  of 


232 


rK   PONTIBUS. 


covei-plules  for  top  chords  imd  inclined  end  posts,  where  the 
limii  may  be  increased  to  forty  (40)  times  the  thiciiness. 
Where  webs  are  built  of  two  or  more  thicknesses  of  plate, 
the  rivets  that  are  used  solely  for  making  the  several  thick* 
nesses  act  as  one  plate  shall  in  no  case  be  spaced  more  than 
(12)  inches  from  each  other,  or  from  other  rivets  connectiog 
said  component  thicknesses  together.  Tiie  least  allowable 
thickness  for  such  compound  web-plates  shu'l  be  one  (1)  inch. 

The  open  sitlis  of  all  compression-members  composed  of 
two  rolled  or  built  channels,  with  or  without  a  cover-plate, 
shall  be  stayed  by  llf-plates  at  inds  and  by  diagonal  lacing  bars 
or  lacing  angles  at  intei  niL'diate  points.  Lacing-bars  may  be 
couneclod  to  the  tianges  by  eilh.'r  one  or  two  rivets  at  each 
end  ;  but  lacing  angles,  which  are  used  for  members  of  lieavy 
section  only,  must  be  (;onnecte(l  by  two  rivets  at  eacli  end. 

The  tie-plates  shall  be  placed  as  close  as  practicable  to  the 
ends  of  the  coniprcssiou-niembcrs.  Their  thickness  shall  not 
be  less  than  one  tifiieth  (Ko)of  the  distance  between  the  centre 
lines  of  the  rivets  by  which  tliey  are  connected  to  the  flanges, 
unless  said  tie  platen  be  well  stiffened  by  angles,  in  which  case 
they  may  be  made  as  thin  as  three  eighths  (!|)of  an  inch. 
The  leiigtii  of  a  tie-plate  shall  never  be  less  than  its  wi»lth,  or 
one  and  one-half  (1|)  times  the  least  dimension  of  strut  (un- 
less it  be  (;lose  io  a  well  diaphragm  of  the  member,  in  which 
case  it.  may  be  lUiwie  n-  short  as  twelve  (13)  inches),  and 
seldom  grenicr  than  one  ami  one  half  (1^)  times  its  width. 

Tlie  thicknesses  of  lacing-bars  shall  never  be  less  than 
one  tiflielh  (B'o)of  the  length  between  centres  of  the  end  rivets, 
measuring  between  inmost  rivets  in  case  that  there  be  more 
than  one  rivet  at  each  end. 

The  smallest  section  for  a  lacing-bar  shall  be  one  and  three- 
quarter  (If)  inches  by  five  sixteentlis  (/j)  of  an  inch,  which 
size  may  be  used  for  channels  und*  r  eight  (8)  inciies  deep; 
and  tiie  largest  section  shall  be  tW"  and  a  half  (2^)  inches  by 
seven-sixteenths  (/ff)  inch,  whieli  size  shall  be  used  for  chan- 
nels fifteen  (15)  inches  deep.  F<m  intermediate  sizes  of  chan- 
nels, the  sizes  of  lacing-bars  shall  be  interpolated.  For  all 
built  channels  of  greater  depth  than  hfteen  (15)  inches,  and 


SPKOIPIOATIO.VS  VOR  STKEL  HIGHWAY  BRIDGES.    3;$:} 


for  all  cases  where  a  laciiij^-^ar  would  require  a  greater  thick- 
ness than  seven  sixteenths  (j^j)  of  an  inch,  angle  lacing  is  to  be 
tised,  the  smallest  section  for  same  being  2"  X  8J"  X  jt",  and 
the  largest  2i"  X  3J"  X  f". 

In  general,  the  iiicliualiou  of  lacing-bars  to  axis  of  member 
shall  be  about  sixty  (GO)  degrees  ;  but  for  members  of  minor 
importance  the  said  inclination  may  be  made  slightly  flatter. 

Pin-plates  shall  be  used  al  all  pinholes  in  built  members, 
for  the  double  purpose  of  reinforcing  for  the  metal  cut  away 
and  reducing  the  intensify  of  pressure  on  pin  and  bearing  to  or 
below  the  specified  limit.  They  shall  be  of  such  size  as  to 
distribute  properly,  through  ihe  rivets,  the  pressure  carried  by 
such  plates  to  both  flanges  and  web  of  each  segment  of  the 
member,  and  shall  extend  at  least  six  (6)  inches  within  the 
tie-plates  of  said  member,  so  as  to  provide  for  not  less  than 
two  (2)  transverse  rows  of  rivets  tliere. 

When  the  pin  ends  of  compression-members  are  cut  away 
info  jaw-plates  or  forked  ends,  for  the  purpose  of  packing 
closely  the  various  members  connected  by  the  pin,  these  jaw- 
plates  or  ix)3t  extensions  shall  be  considered  as  columns,  the 
thickness  of  each  of  which  shall  be  determined  by  the  follow- 
ing formula: 


p  =  10,000 


300^-; 


where  p  is  the  greatest  allowable  intensity  of  working-stress 
(impact  being  considered);  I  is  the  unsupported  length  in 
inches,  measuring  from  the  centre  of  the  pinhole  to  the 
centre  of  the  first  transverse  line  of  rivets  beyond  the  point  at 
which  the  full  section  of  the  member  begins;  and  t  is  the  total 
thickness  in  inches  of  one  j;iw.  Tlie  length  I  is  always  to  be 
nuule  as  small  ns  practicable,  and  in  cases  of  unavoidably  long 
exieusions  the  plates  are  to  be  stiffened  by  an  interior  dia- 
phragm composed  of  a  web  with  four,  or  sometimes  only  two, 
angles. 

It  is  always  better,  whenever  practicible,  to  avoid  cutting 
away  the  ends  of  cliannels  ;  but   if  iliey  must  be  trimmed,  the 


234 


i)K  poNTinrs. 


euds  must  be  reinforced  s'o  Uiiit  the  strength  of  tlie  member 
shall  not  be  reduced  by  the  Iriniming, 

In  riveted  teusion  members  tlie  net  section  throu^;li  any  pin- 
liole  shall  have  an  ari'a  fifty  (50)  per  cent  in  excess  of  tlu,'  nel 
sectional  area  of  the  body  of  I  he  member.  The  net  section 
outside  of  the  pinhole  along  llie  centre  line  of  stress  shall  be 
at  least  sixty-five  (05)  per  cent  of  the  net  section  through  the 
pinhole. 

Pius  are  to  be  i)ioportioned  to  resist  the  greatest  shearing 
and  bending  produced  in  them  by  the  bars  or  struts  wliich 
they  connect.  No  pin  is  to  have  a  diameter  less  than  eight 
tenths  (^''(j)  of  the  depth  of  the  deepest  eye-bar  coupled  thereon. 
No  pin  is  to  have  a  smaller  diameter  than  two  and  a  half  (2^) 
inches. 

Lower  chords  are  to  be  packed  as  closely  as  possible,  and  in 
such  a  manner  as  to  produce  tlie  least  bending  moments  on  the 
pins  ;  but  adjacent  eye-bars  in  the  same  panel  must  never  iiavc 
less  tha,'  one-half  (A)  inch  space  between  them,  in  order  to 
facilitate  j^aintiug.  The  various  members  attaclied  to  any  pin 
must  be  pac\ed  as  closely  as  practicable,  and  all  interior  vacant 
spaces  must  be  tilled  with  steel  fillers,  where  their  omission 
would  pern  it  of  motion  of  any  member  on  the  pin.  All  bars 
are  to  lie  in  planes  as  nearly  as  possible  parallel  to  the  central 
truss  plane. 

In  detailing  I  struts  composed  of  four  angles  with  a  single 
line  of  lacing,  llie  clear  distance  between  backs  of  angles  shall 
never  be  made  less  than  three  quarters  (|)  of  an  inch,  in  order 
to  permit  the  insertion  of  a  small  paint-brush. 

The  greatest  allowable  pressure  upon  expansion-rollers  of 
fixed  spans,  when  impact  is  considered,  shall  be  determined  by 
the  equation 

p  =  mod, 

where  p  is  the  permissible  pressure  in  pounds  per  lineal  inch 
of  roller,  and  d  is  tlie  diameter  of  tlie  latter  in  inches.     Tlie 
least   allowable  diameter  for  expansion-rollers  is  two  and  a 
quarter  (3J)  inches. 
Rollers  shall  be  enclosed  in  boxes  made  practically  dust- 


SPKClPlCATtONS  FOR  STRML  MIOIIWaV  f.MDG^r..  235 


of 


tight,  but  which  will  not  ri't.aiii  water,  aiid  which  are  so  de- 
signed that  the  sides  can  be  readily  removed  for  the  purposj 
of  cleaning.  These  boxes  must  be  so  designed  us  to  permit  of 
the  free  niovemenl  of  the  rollers  in  the  longitudinal  direction 
of  span  sufficient  to  take  up  the  extreme  variations  in  leugtli 
due  to  temperature  changes  and  deflection,  and  at  the  sjune 
time  prevent  any  transverse  motion  of  the  end  of  span. 

All  shoe-plales,  bed-plates,  and  roller-plates  are  to  be  so 
stiffened  that  the  extreme  fibre-stress  under  bending,  when 
impact  is  included,  shall  not  exceed  sixteen  thousand  (16,000) 
pounds  per  sipiare  incii. 

Pedestals  shall  be  either  of  cast  steel  or  built  up  of  plates 
and  shapes.  lu  buill  pedestals,  all  bearing-surfaces  of  the 
base  plates  and  vertical  bearing-plates  must  be  planed.  The 
vertical  plates  nuist  be  secured  to  tlie  base  by  angles  liaving 
at  least  two  rows  of  rivets  in  the  vertical  legs;  and  the  said 
vertical  plates  must  boar  properly  from  end  to  end  upon  said 
base.  No  ba><e- plate,  vertical  plate,  or  connecting  angle  shall 
be  less  than  five-eighths  (|)  of  an  inch  in  thickness.  The  verti- 
cal plates  shall  be  of  sufflcieiit  height,  and  must  contain 
enough  metal  and  rivets  to  distribute  properly  the  loads  over 
the  bearings  or  rollers.  The  bases  of  all  cast-steel  pedestals 
shall  be  planed  so  as  to  bear  properly  on  the  miusoury  or 
rollers. 

All  rollers  and  the  faces  of  base-plates  in  contact  therewith 
are  to  be  planed  smooth,  so  as  lo  furnish  perfect  contact  be- 
tween rollers  and  plates  throughout  their  entire  length. 

Heads  of  eye-bars  are  to  be  made  of  such  dimensions  that, 
when  the  bars  are  te-ited  to  destruction,  they  shall  break  in  the 
body  and  not  in  the  eyes;  and,  in  the  case  of  loop  eyes,  so 
that  they  shall  not  fail  in  the  welds.  Rods  with  bent  e}  es 
shall  not  be  used.  In  loop  eyes,  the  distance  from  the  inner 
point  of  the  loop  to  the  centre  of  the  pinhole  must  not  be  less 
than  two  and  one  half  (2i)  times  the  diameter  of  the  pin,  and 
the  loop  must  fit  closely  to  the  pin  throughout  its  semi-cir- 
cumference. 


fine 


l>K   I'ONTIBUS. 


DETAILS  OF  DESIGN  FOR  VIADUCTS. 

Tlie  specifications  for  ilie  "  Dctiiils  of  Design  for  Trestles 
and  Elevated  Riiilroads"  are  in  general  to  be  followed  as  far 
as  lliey  will  apply  in  llie  designing  of  highway  viadiicis,  l lie 
]>rincipal  variation  being  that,  for  clieap  structures,  adjustable 
rods  with  clevises  nviy  b  •  substituted  for  tiie  stiff  diagonals  in 
tile  four  faces  of  the  braced  towers,  by  adding,  of  course,  hor- 
izontal struts  at  the  panel  points  of  the  transverse  and  longi 
tudinal  bracing.  The.se  struts  must  be  riveted  to  the  columns 
by  means  of  wide  plates  to  which  the  clevises  at'ach,  and 
must  never  be  pin- connected.  Corner  horizoital  plates  are  to 
be  employed  for  attaching  the  horizontal  adjustable  rods  by 
means  of  clevises,  each  of  said  plates  being  riveted  to  both  a 
transverse  and  a  longitudinal  bracing  strut. 

The  detailing  for  tlie  loiigitudiiuil  girders  of  viaducts  and 
the  bracing  between  same  shall  comply  with  the  specifications 
for  detailing  highway  plate  or  open -webbed,  riveted  girder 
spans;  and  the  specifieat  >hs  for  wooden  floor  system,  paving, 
hand  rails,  etc..  shall  be  the  same  for  highway  viaducts  as  for 
highway  bridges. 


CIIAPTEIi  XVII. 

SPECIFICATIONS  FOR  HIGHWAY   DRAW-SPANS. 

Thkse  s|)eciflcation3  will  be  given  i)riiicipall3'  l)y  reference 
lo  the  previous  speciflciitions  for  Railnmd  Structures,  High 
way  Bridges,  and  Railroad  Draw-Si)ans. 

OENEBAL  DESCRIPTION. 
CLASSIFICATION. 

See  Speciticalioud  t\>L-  Highway  Bridges. 

MATERIALS. 

See  Speciflcatious  for  Railroad  Dra\v-.>5pans. 

JOISTS,    PLANKS,    GCAKD-TIMIiERS,    AND   WOODEN 
HAND-RAILS. 

See  Speciflcatious  for  Highway  Bridges. 

FLOORING   ON   APrUOAClIES. 

See  Specifications  for  Highway  Bridges. 

HTEEL   RAILROAD    I'HACKS. 

See  Specifications  for  Highway  Bridges. 

PAVED  FLOORS. 

See  Specifications  for  Highway  Bridges. 

CLEARANCES. 

See  Specifications  for  Highway  Bridges 

237 


238 


PE    TON TI BUS. 


EFFECTIVE   LENGTHS   AND   DEPTHS. 

See  Specifications  for  Hiiilruiul  Structures. 

8TYI.ES   OF   H1UI)(!KS   FOU   VAHIOIIS   SPAN    liKNGTHS. 

For  spans  up  to  one  luimlred  iind  forty  (140)  feet  in  length, 
plate-ginicr  spans  are  to  be  used.  Thehc  plate-girder  spans 
may  be  made  to  act  as  continuous  girders  over  tlie  pivot-pier, 
or  may  have  pin-conntclions  over  the  drum,  so  that  when  the 
live  load  is  applied  they  will  act  as  two  separate  spans.  The 
former  style  is  generally  i)referable  as  a  matter  of  economy  in 
time  of  operation,  there  being  no  important  reason  for  raising 
the  ends  to  any  great  extent,  as  there  is  in  the  case  of  railroad 
diaw-spans. 

For  spans  hetween  one  hundred  and  forty  (140)  and  two 
hundred  and  twenty-five  (225)  feet,  pin-connected  Pratt 
trusses  with  parallel  chords  are  to  be  used. 

For  spans  between  two  hundred  and  twenty-five  (225)  feet 
and  three  hundred  (300)  feet,  pin-connected  Pratt  trusses  with 
broken  top  chords  are  to  be  employed. 

For  spans  of  over  three  hundred  (300)  feet,  pin-connected 
trusses  witli  subdivided  panels  are  to  be  adopted. 

It  is  understood  that  these  limiting  lengths  are  not  fixed 
absolutely,  ns  the  best  limits  will  vary  somewhat  with  the 
width  of  bridge  and  the  live  load  to  be  carried. 

The  proper  truss  depths  for  all  ca.ses  cannot  well  be 
specified,  as  tliey  will  depend  upon  various  considerations, 
such  as  appearance,  economy,  width  of  structure,  etc. 

In  all  cases  the  top  chords  are  to  be  of  rigid  members,  and 
inclined  posts  are  to  be  used  at  ends  and  over  drum,  as 
specified  for  railroad  draw-spans. 


MAIN   MEMBERS  OF  TRUSS   DRAW-SPANS. 

See  Specifications  for  Highway  Bridges. 

LOADS. 

See  Specifications  for  Railroftd  Draw-Spans. 


SPKCIFICATIONS   FOR    HKillWAV    DUA WSl'ANS     )iii\) 


LIVK    LOAUB. 

See  Specifications  for  Iligluvay  Uridines ;  imd,  for  llio 
manner  «)f  applying  live  loads  to  (Iraw-spans,  see  Specificatioua 
for  Hailroad  Draw-Spans. 

IMPACT- A  LLOWANCK   LOAD. 

See  Specifications  for  Highway  Bridges. 

DKAI)    LOAD. 

See  Specifications  for  Railroad  Structures. 

ASSIIMKD   UPLIKT   HTHESSKH. 


See 

Specifications  for  Itiiiiroad  Draw-Spans 

. 

Tlie 

inferior  limit  of  uplift  for  designing  tlie  niacliinery  of 

ligl.t 

iiigliway  drawbridges  is   to  be    taken 

at  ten  thousand 

(10,000)  pounds  at  each  of  the  four  corners  o 

'  the  span. 

"WIND   LOADS. 

See 

Specifications  for  Highway  Bridges. 

For  method  of                     s 

using 

tlic  wind  loads,  see  Specifications  for  Railroad  Draw-                     j 

Spaus 

• 

INDIRECT   WIND   LOAD   On   TTtANHFRHKED   LOAD.                                             * 

See 

Specifications  for  Railroad  Structures. 

For  method  of 

dealing  with  this  load,  see  Specifications  for 

liiiilroad  Draw- 

Spans 

INTENSITIES    OP   WORKING-STRESSES. 

See 

Specific  aious  for  Railroad  Structures. 

Bh AMINOS   UPON    MASONUY. 

See 

Specifications  for  Railroad  Structures. 

REVEHSINO   STRESSES. 

See  Speciticalious  for  Itailroad  Structures. 

NET   SECTION. 

See 

Specifications  for  Railroad  Structures. 

_< 

240 


DE    I'ONTIHUS. 


BENDING   MOMKNT8  ON   PINS. 

See  Speclficulious  for  liiiilrotul  Structures. 

COMIU NATIONS  OK   STUE88E8. 

See  Specitications  for  Riiilroad  Draw -Spans. 

It  is  to  be  observeii,  liowever,  that,  for  spans  which  do  not 
carry  trains,  tlie  live  load  and  the  wind  load  are  ftssumed  not 
to  act  simultaneously. 

KENUING   ON   TOP  CHORUS. 

See  Specifications  for  Railroad  Structures. 

BENDINO   ON   INCLINED  KN'D   P08T8. 

The  Specifications  for  Railroad  Structures  under  this  head- 
ing are  to  he  followed  with  this  exception  :  in  bridges  that  ilo 
not  carry  trains,  the  live  load  and  the  wind  loail  are  assumed 
not  to  act  simultaneously. 

BENDING   DUE  TO   WEIGHT  OF   .MEMBEH. 

See  Specifications  for  Railroad  Structures. 

OENERAIi  LIMITS   IN  DESIONINO. 

See  Specifications  for  Highway  Bridges. 

GENERAL  PRINCIPLES   IN    DESIONINQ. 

See  Specifications  for  Railroad  Structures. 

RIVETING. 

See  Specifications  for  Highway  Bridges. 

DETAILS  OF  DESIGN  FOR  PLATE-GIRDER  DRAW- 
SPANS. 

The  specifications  for  the  corresponding  item  in  tije  Specifi- 
cations for  Railroad  Draw-Spans  are  to  be  followed,  with  the 
following  exceptions : 

Ist.  The  perpendicular  distances  between  central  planes  of 
girders  will  be  made  to  suit  the  ^    leral  requirements;  and. 


IIK 


Si 


SPECIFICATION'S    FOIt    HI(ii..VAV    l>l{A\V-SI'A  NS.    '341 

2(1.  Al  least  eiglit  (8)  poiiils  of  support  on  the  drum  will 
be  needed. 

DETAILS   OF    DESIGN   FOR   PIN-CONNECTED 
DBAW-SPANS. 

The  specifications  for  Ihe  correspond inj?  item  in  tlic  Spcci- 
licalions  for  Hifj;iiw:iy  Bridij:c.s  are  to  be  followi  d,  and  in  addi- 
lion  tliurcto  tiioso  given  umior  the  Inwling  "  Details  of  Des'gn 
forTrnssesof  Draw-Spans"  in  llic  Specilieiitions  for  Kailroad 
Draw-Spans  are  to  bo  employed,  except  that  tiie  use  of  adjust- 
able members  for  lateral  diagonals  will  be  permitted  in  the 
case  of  cheaj)  highway  draw-spans. 

DETAIL.S   OF   DRUMS   AND   TURNTABLES. 

Ill  general  the  Speeilicalions  for  the  corresponding  item  in 
the  Specifications  for  Railroad  Draw-Spans  sliall  be  followed, 
ex(!ept  that,  for  liglit,  highway  draws,  tlie  limiting  thicknesses, 
etc.,  may  be  reduced  to  the  following  : 

Top  flanges  and  webs  of  drums— three  eighths  (|)  of  an 
iiicii. 

Bottoffk  flanges  of  drums— five  eighths  (f)  of  an  inch. 

Upper  track  segments — one  and  three-cpiarter  (IJ)  inches. 

Lower  tr.ick  segments — two  (2)  inches. 

B:'aring-plates  over  drum — three  (piarters  (})  of  an  inch. 

Centre  casting  on  pivot-pier — one  (1)  in(!h. 

Anchor-bolts  for  same — one  and  one-eighth  (IJ)  inches  in 
diameter  and  two  and  a  half  (2i)  feet  long. 

Rollers — ten  (10)  inches  in  diameter  and  six  (6)  inches  face. 


MACHINERY  FOR  TURNING  THE  SPAN  AND  LIFTING 
THE  ENDS  OF  SAME. 

See  Specifications  for  Railroad  Draw-Spans. 


METHOD  OF  DETERMINING  POWER  REQUIRED  FOR 
OPERATING  THE  SPAN  AND  LIFTING  THE  ENDS. 

See  Specifications  for  Railroad  DiawSpans, 


242 


DE   PONTIBUS. 


DETAIIiS    OF   MACniNEBY. 
Ol'ti'*  i'lNO   MACIIINEIIY. 

See  Specifications  for  .<ttilroud  Diaw-Spaus. 

END-MFTING   AFPAUATU8. 

See  Specificatious  for  Ilailroiul  Dniw-Spans. 

SHOES  AND   END  BEARING   UOLLEUS. 

See  Specifications  for  Railroad  Draw -Spans. 

HOU8EH   AND  HUl'I'OHTS. 

See  Specifications  f.-  Railroad  Draw-Spans. 

CAMBER  AND    DEFLECTION. 
See  Specifications  for  Railroad  Draw-Spaus. 


CHAPTER  XVIII. 

OENERAL  SPECIFICATIONS  (JOVKRXINfJ  THE  MANUFACTURE, 
SHIPMENT,  AND  ERECTION  Oh'  STEEL  BRIDGES,  TRESTLES, 
VIADUCTS,   AND  ELEVATED  RAILROADS. 

DRA'    TNOfl. 

As  soon  as  practicable  after  the  signing  of  the  contract  for 
building  tlie  structure,  cuiiiplete  detail  drawings  will  be  fiir- 
uislied  by  llie  Engineer,  and  from  these  the  Contractor  is  to 
prepare  his  shop  drawings,  complying  can.'fidly  therewith 
and  making  no  changes  without  tl)e  writlcn  consent  of  tlie 
Engineer.  The  working  drawing:t  are  to  be  sent  in  triplicate 
for  tne  ai)provaI  of  the  Engineer  and  his  principal  Shop 
Inspector,  who  'vill  retain  two  sets  and  return  the  tliird  after 
clieckiug  same  and  marking  thereon  any  changes  or  conec 
tions  desiieii  ;  after  which  a  correc:ted  set  of  shop  drawings 
shall  be  sent  without  delay  by  tlie  Contractor  to  the  Engineer. 
The  appioval  o'  said  working  drawings  by  the  Engineer  will 
not  relieve  the  Contractor  from  tlie  responsibility  uf  any  errors 
thereon. 

Tiie  drawings  furnished  b}'  the  Engineer  shall  be  checked 
carefully  by  the  Contractor  before  beginning  work.  Should 
any  errors  be  di.scovered,  the  Engineer's  attention  shall  be 
called  to  .same,  and  corrections  will  be  made,  after  whicli  the 
Contractor  sludl  be  responsible  for  all  errors  which  may  occur 
or  which  may  have  occurred.  The  Engineer  shall  have  the 
right  to  alter  as  he  may  see  hi  Mie  preliminary  plans,  if 
further  investigation  of  the  conditions  affecting  the  proposed 
structure  so  warrant ;  and  he  shall  be  at  liberty  to  make 
minor  changes  iu  all  plans  during  coDStructiou  without  any 

848 


J.M4 


I)K    I'ONTinUS. 


extra  charge  for  siiine  being  lUiulc  by  the  Coiitniclor  unless, 
iu  the  opinion  of  the  Eiigiiicer,  the  Contractor  be  really  en- 
titled to  extra  conipens  ition  on  account  of  such  ciianues. 

The  Conlraclor  shall  furnisii  without  cluirge  us  many  sets 
of  shop  drawings  as  the  Engineer  and  other  ofticers  of  tiie 
company  may  deem  necessary  for  iheir  use  during  const riic- 
tioa  or  for  record. 

INSPECTION. 

The  inspection  and  tests  of  metal  will  be  made  promptly  on 
its  being  rolled  or  cast,  and  the  (jualiiy  will  be  deleimined 
before  it  leaver  the  rolling-niiil  or  fouiulry  The  inspection 
of  workmanship  will  be  made  as  tiie  manufacture  of  the 
material  progresses,  and  at  as  early  a  period  as  the  nature  of 
the  work  will  permit. 

All  facilities  for  inspection  of  matcnial  and  workmanship 
shall  be  furnished  l)y  the  Contractor  ;  and  ihe  Engineer  and 
his  inspectors  shall  have  free  access  to  any  of  the  works  in 
which  any  portion  of  the  material  is  being  made. 

The  Contractor  shall  give  the  Inspector  due  notice  when 
any  material  is  ready  for  inspection.  Any  delay  on  tlie  part 
of  the  Inspector  .shall  be  reported  to  the  Engineer,  but  no 
nuiterial  will  be  accepted  which  has  not  been  passed  tipon  by 
the  authorized  representative  of  the  Engineer. 


.MKTAT.. 

Unless  otherwise  specified,  all  metal  shall,  for  all  spans  of 
ordimiry  length,  be  medium  steel  ;  excepting  only  that  rivets 
and  bolts  are  to  be  of  soft  steel,  and  adjustMble  members  of 
either  soft  steel  or  wrouglit  iron. 

For  vtry  long,  fixed  sivms,  high  steel  may  lie  used  for  top 
chords,  inclined  end  posts,  pins,  and  eye-bars  in  bottom  choids 
and  in  nwiiii  diagonals  of  panels  where  there  is  no  reversion 
of  stress,  when  impact  is  in(;luded.  It  may  be  u.sed  al.so  for 
the  web  members  of  cantilever  and  anchor  arms  of  cantiUtver 
bridges,  where  the  variation  of  stress  is  comparatively  small, 
au<l  where  the  impjict  cannot  be  great, 


grei 
in 


Snip 
Silic. 
i'Miirii 


SPHCri'-ICATIONS    FOR   STKKL    IN    lUilDUKS,   KTC.    '^4') 

Except  for  purely  oriiameiilal  work  and  ii  few  iiihior  details 
of  tlio  operating  machinery  of  draN.bridges,  cast  iron  will  not 
be  allowed  lo  be  used  in  the  .superstructure  of  any  bridge, 
trestle,  viaduct,  or  elevated  railroad,  cast  steel  being  em- 
ployed wherever  important  castings  are  necessary. 

KOLLEl)   STEEl/. 

All  soft  and  medium  steel  shall  bo  manufactured  by  either 
the  acid  or  the  t)asic  openhearlh  process,  but  high  steel  shall 
invariabl}'  be  uiaiiufactured  by  tlic  former.  All  steel  must 
be  uniform  in  ch;H;icter  for  (,'ach  specified  kiiul.  Any  attempt 
lo  sul)stitule  Bes.semer  or  any  other  sicel  for  the  open-hearth 
product  will  be  considered  as  a  violation  of  the  contract  and 
a  good  and  sufHcienl  reason  for  cancelling  the  same. 

All  plates  shall  be  rolled  from  slabs.  These  slabs  shall  be 
made  by  a  sei>arale  oi)eralion,  by  rolling  an  ingot  and  cutting 
off  the  scrap.  The  originid  ingot  shall  have  at  least  twice 
the  cross-sectional  area  of  the  slab,  and  the  latter  shall  be  at 
least  six  times  as  thick  as  the  plate. 

All  finished  material  coming  from  the  mills  must  be  free 
from  seams.  Haws,  or  cracks,  and  must  have  a  clean,  smooth 
tinish. 


COMPOSITION   OK   ROIXRD   BTEEL. 

The  greatest  allowalile  percentages  of  certain  principal  in- 
gredients of  the  various  kinds  of  rolled  steel  shall  be  as  given 
in  the  following  table: 


Iiigredi^iit.^. 

Percentages. 

Soft  Steel. 
0  0.5 

0  ();i 

0.04 
0.04 
0.00 

Medium  Steel. 

Hl«li  Steel, 

T'hoHphoniR  (acid  steel). . 
I'liosplioiiis (Imslc  steel). 

Sulphur 

Silicon 

JiutiKanese 

O.OC 
0.04 
0.05 
0.05 
0.70 

0.07 

005 
0.06 
0.80 

34G 


Dli    PONTlBfS. 


These  percentages  apply  to  drillings  taken  from  the  edges 
of  plates  and  tlie  exterior  of  shapes,  bars,  or  Hals.  If,  how- 
ever, the  drillings  be  taken  from  tlie  middle  of  plates  or  the 
heart  of  other  sections,  the  percentages  given  in  the  table  are 
to  be  increased  twenty-flve  (25)  per  cent. 

IDENTIFICATION. 

Each  ingot  shall  be  stamped  or  marked  plainly  with  its 
proper  melt-number;  and  this  melt-number  must  be  stamped 
or  pamted  plainly  on  all  blooms,  billets,  or  slabs  made  from 
such  ingots  in  order  to  identify  the  material  throughout  its 
various  processes  of  manufacture;  and  the  melt-numbiT  must 
be  stamped  plainly  on  each  piece  of  linishcd  material.  Rivet 
and  lacing  steel,  and  small  pieces  for  pin-plates  and  stiffeners, 
may  be  shipped  in  bundles,  securely'  wired  together,  wiili  the 
i)low  or  ■melt  number  on  a  metal  tag  attached. 


W' 


OKNKKAI.   PKOVISIONS  ON   MKTUODS   oK   TKsriNO. 

Rivet-rods  and  other  rounds  are  to  l)e  tested  in  the  form  in 
which  they  leave  the  rolls,  without  machining. 

Test-pieces  from  angles,  plates,  shapes,  etc.,  shall  be  rect- 
angular in  shape,  with  a  cross-sectional  area  of  preferably 
about  one  half  (i)  of  a  .square  inch,  but  not  less,  and  shall  be 
taken  so  that  only  two  sides  are  machine  fiaislieil,  the  other 
two  having  the  surface  which  was  left  by  the  rolls. 

Should  fracture  occur  outside  of  the  middle  third  of  the 
gauge  length,  the  test  is  to  be  discarded  as  worthless  if  it  falls 
below  the  .standard., 

If  any  test-piece  Itave  a  manifest  flaw,  its  test  shall  not  be 
considered. 

In  case  that  one  test-piece  falls  .slightly  below  the  require- 
ments in  any  particular,  the  Inspector  nuvy  allow  the  re-testing 
of  the  lot  or  heat  by  taking  four  (4)  addilioind  tests  from  the 
.said  lot  or  heat;  and,  if  the  average  of  the  five  (5)  shall  show 
that  the  steel  is  within  the  requirements,  the  metal  nniy  l)e 
accepted;  otherwise  it  shall  be  rejected. 


te 
in 


■' 


SPRCrFIOATIONS   FOK  STEEL   IN   BRIDGES,    ETC.    247 

Drillings  for  ciiemical  analysis  may  be  taken  eitlier  from  tlie 
preliminary  test-piece  or  from  the  finished  malerial. 

The  speed  of  the  machine  for  breaking  test-pieces  shall  not 
be  less  than  one-qiiarler  (J)  inch  per  minute,  nor  more  than 
three  (8)  inches  per  mlniile. 

Material  wliich  is  to  be  used  witliout  annealing  or  furliier 
trL'atnu'nt  is  to  b(^  tested  in  the  condition  in  which  it  comes 
from  the  rolls.  When  thcmnterial  is  to  i)<;  annenled  or  ()liier- 
wise  treated  before  use,  the  specimens  representing  such  ma- 
terial maybe  similarly  treated  before  testing;  but  they  shall 
also  give  standard  elongation,  reduction,  and  fracture  before 
annealing. 

TENSILE   STRENGTH. 

The  ultimate  tensile  strenglii  per  square  inch  on  test-pieces 
for  all  tliree  kindsof  rolled  sled  used  in  structiual  metal-work 
shall  be  as  follows  : 

Soft  steel r)0,()00  lbs.  to  00.000  lbs. 

Medium  steel  .  . .   60,000  lbs.  to  70,000  lbs. 
High  steel 70,000  lbs.  to  80,000  lbs. 


not  be 


ELASTIC   MMITS. 

The  least  allowable  elastic  limits  obtained  from  test-pieces 
and  determined  in  the  usual  manner  by  the  dro[)  of  the  beam 
shall  be  as  follows  : 

Soft  steel 30,000  lbs  per  .square  inch. 

y    Mum  steel...   3"),  000  lbs.  per  square  inch. 
High  steel     40,000  lbs.  per  square  inch. 

ELONGATION. 

The  percentages  of  elongation  sliall  be  obtained  from  the 
test-pieces  after  l)rcakiug  t)ii  an  original  length  of  eight  (8) 
inches,  in  which  length  must  occur  the  curve  of  reduction 
from  stretch  on  both  sides  of  the  point  of  fracture.  The  least 
allownlile  elongations  for  the  various  kinds  of  rolled  structuial 
steel  shall  be  as  follows. 


348 


IJE    I'ONTIHUS. 


Percentage  of  Elongation. 

Shape. 

Soft  Steel. 

IMediiini 
Steel. 

High 
Steel. 

Roiiuds  (exceDtin^  Dins) 

29 
29 

27 
20 
26 
24 

23 
21 
20 

Pins 

18 

Angles  antl  bars 

22 

Plates  under  40"  wide 

Plates  40"  to  70"  wide,   and    webs 
of  beams  and  channels 

20 
19 

Plates  over  70"  wide 

Flanges  of  beams  and  ciiannels 

18 

UliUUCTIO.N   OK   .\KE.\. 


Tiie  reduction  of  area,  inuii.surc'd  on  test-pieces,  for  the  va- 
rious kiuds  of  rolled  slruc'.ural  steel  shall  be  as  follows : 


Percentage  of  Reduction  of  Area. 

Shape. 

Soft  Steel. 

Medium 
Steel. 

High 
Steel. 

Rounds  (exceutlne  nins) 

50 

48 

44 

40 
40 
40 

38 
37 
86 

88 

Pins 

Angles  and  bars 

34 
34 

Plates  under  40"  wide 

Plates  40"  to  70"   wide,   and   wel)s 

U 
34 

Plates  over  70"  wide 

Flanges  of  beams  and  channels 

30 

HEND1NG  TE8T8. 

Spcciineiis  of  soft  steel  shall  be  capable  of  bending  to  one 
hundrfd  and  ciglily  (IHO)  dciirees  and  closing;  down  Mat  u[)on 
tlicinsulvcs,  without  cracking,  when  either  liot,  cold,  or 
qiteuchcd. 

Specimens  of  mediiitn  steel,  when  heated  to  a  dark  orange 
nud  cooled  in  water  at  .seventy  (70)  degrees  Fahrenheit,  or 
when  cold  or  hoi,  shall  be  capable  of  bending  one  hundred 
and  eighty  (180)  degrees  around  a  circle  whose  diameter  is 
equal  to  llic  tliickncss  of  the  test-piece,  without  showing  signs 
of  cracking  on  the  convex  side  of  tlie  bend. 


SPKCJIFICATIUNS    h'OU   STEKF.    IN    BliIJKiE:S,   ETC.    ;.'49 

Specimens  of  liigh  sleel  when  qiieiielied  in  a  siniiliir  rniuincr 
shiill  be  ciipuble  of  l)i'n(ling  ninety  (90)  degrees  around  a  circle 
whose  diameter  is  equal  to  twiee  the  thickness  of  the  test-piece, 
and  one  hundred  and  eiglity  (180)  degrees,  either  hot  or  cold, 
without  showing  signs  of  cracking  on  the  convex  side  of  the 
bend. 

UKIFTINO   TESTS. 

Punched  rivet-lioles  in  medium  steel,  pitched  two  (2)  diam- 
eters from  a  slieared  edge,  nuist  stand  drifting  until  their 
diameters  are  fifty  (50)  per  cent  greater  than  those  of  the 
original  holes,  and  must  show  no  signs  of  cracking  the  metal. 

Higii  steel  must  stand  the  same  test,  except  that  the  increase 
in  diameter  is  to  be  twenty-five  (25)  per  cent  instead  of  fifty 
(50)  per  cent. 

FUACTURE. 

All  broken  test-pieces  for  all  three  classes  of  steel  must  show 
a  silkv  fracture  of  uniform  color. 


NUMBKK   OF  TEST- PIECES. 

At  least  three  (3)  tensile  tests  and  three  bending  tests  shall 
be  made  on  specimens  tvom  dilTc  rent  ingots  of  each  melt. 
The  bending  tests  mny,  if  desired,  be  in;ide  on  the  broken 
test-pieces  of  the  tension  tests.  If  material  of  various  shapes 
is  to  be  made  from  the  same  melt,  the  specimens  for  testing 
are  to  be  so  selecteil  as  to  represent  the  dilfereut  shapes  rolled 
from  such  melt. 

All  tests  are  to  l)e  made  by  the  Contractor  for  the  In.spector 
without  charge. 

The  Insi)ector  will  Ix;  permitted  considerable  latitude  in 
respect  to  the  number  of  tests  re(iuired,  reducing  same  when 
the  metal  runs  uniforndy  and  increasing  .same  when  it 
does  not. 

Lots  for  testing  .shall  not  exceed  twenty  (30)  tons  in  weight; 
and  plates  rolled  in  universal  mill  or  in  grooves,  or  sheared 
plates,,  shall  each  constitute  a  separate  lot,  as  shall  also  the 
angles,  channels,  or  beams. 


250 


T)K   I'ONTIHUS. 


TESTS  OF   FUtiL-SIZED   ETE-BARS. 

Full-sized  eye-bars  may  be  tested  to  destruction ,  provided 
notice  be  given  in  advance  of  the  number  and  size  required  for 
this  purpose,  so  tliat  the  material  can  be  rolled  at  the  same 
time  as  that  required  for  the  structure.  The  number  of  tests 
of  full-sized  eye-bars  will  depend  upon  the  size  of  the  order 
and  upon  the  reguliirity  of  tlie  results  of  the  tests.  In  general, 
for  small  orders,  the  number  of  tests  shall  be  about  tliree  (3) 
per  cent  of  the  number  of  eye-bars  in  the  order,  but  never  less 
than  two  bars  for  an  order  for  a  single  span.  For  large  orders 
the  number  of  tests  shall  be  about  two  (2)  per  cent  of  the 
number  of  eye-bars  in  the  order.  Should  the  Inspector  find 
the  bars  to  be  very  uniform  in  strength,  elasticity,  and  duc- 
tility, and  fully  up  to  the  specifications,  he  will  be  at  liberty 
to  reduce  the  number  of  tests  of  full-sized  l)ars.  In  the  case 
of  testing  long  bars,  it  will  be  allowable  to  choose  a  bar  at  ran- 
dom from  a  nuiul)er  of  finished  bars,  cut  it  in  two,  and  upset 
the  cut  end  of  eacli  piece,  tlius  making  two  test-bars. 

Full  sized  bars  of  medium  slcjd  must  show  an  ultimate, 
tensile  strength  of  at  least  sixty  thousand  (60,000)  potmds  per 
square  inch  for  bars  of  one  (1)  inch  thickness  and  under,  and 
not  less  than  fifty  six  thousand  156, 000)  pounds  per  stiuare  incii 
for  bars  of  two  (3)  inches  thickness  and  over.  Bars  witli 
thicknesses  betweei  these  limits  must  show  proportionate 
strength.  The  elongation  shall  lie  not  less  than  fourteen  (14) 
per  cent  in  a  gauged  length  of  ten  (10)  feet;  and  the  elastic 
limit  shall  not  be  less  than  fifty-five  (55)  per  cent  of  tiie 
ultimate  strength  of  tlie  bar  for  bars  not  over  one  inch  thick, 
or  less  than  fifty  (50)  per  cent  of  same  for  bars  of  two  (2) 
inches  thickness  and  over,  with  proportionate  percentages  for 
bars  of  intermediiite  thicknesses. 

For  high  steel  the  limits  just  specified  shall  be  changed  as 
follows : 

Ultimate  strength,  70,000  to  65,000  pounds. 

Elongation,  twelve  (1'^)  per  cent. 

Elastic  linut,  fifty-two  (53)  per  cent  to  forty-seven  (47) 
per  cent. 


SPRorPICATroNS   Poll   STRRL   iK    MUIDnES,    ETC.    351 

Any  lot  of  steel  burs  which  meets  the  requirements  of  tlie 
preceding  paragraph  shall  be  accepted,  if  none  of  the  bars 
which  break  in  the  eye  show  an  ultimate  strength,  elastic 
limit,  or  elongation  less  than  that  specified  for  the  body  of 
the  bar,  unless  one  fourth  (J)  of  the  full-sized  samples  so 
tested  break  in  the  eye.  In  case  of  failure  to  meet  any  of 
these  requirements,  the  lot  from  which  the  sample  bars  were 
taken  will  be  rejected. 

All  full-sized  sample  bars  which  break  at  less  than  t'  e 
ultimate  strength  .specified,  or  do  not  otherwise  fill  the  .speci- 
fications, sliall  be  at  liie  expense  of  the  Contractor  ;  luiless,  in 
case  of  tiiose  that  break  in  tlie  eye,  he  siiall  have  made  objec- 
tion in  writing  to  the  form  or  diinen.sio:is  of  tiie  heads  before 
making  the  eye-bars.  All  others  .shall  be  paid  for  by  the  pur- 
chaser at  the  contract  price  of  finisiied  metal-work  on  cars  at 
shops,  less  tlie  scrap  value  of  the  broken  bars. 

PIN   METAh. 

Pins  up  to  si.\  (6)  indies  in  diameter  may  be  rolled,  but 
al)ove  that  diameter  they  shall  be  forged.  The  rounds  from 
which  the  pins  are  to  be  turned  must  be  true,  stiaight,  and 
free  from  all  injurious  Haws  or  cracks.  All  forged  pins  shall 
be  reduced  from  a  single  bloom  or  ingot  until  perfect  homo- 
geneity is  .secured  throughout  tlie  wliole  mass.  Tiie  blooms 
shall  have  at  lea,st  three  (3)  times  tlie  sectional  area  of  the 
finished  pins.     No  forging  shall  be  done  below  a  red  heat. 


VAUIATION   IN    WEIGHT. 

Except  in  the  case  of  sheared  plates  orilered  to  gauge,  a 
variation  in  cross-section  or  weight  of  rolled  material  of  more 
than  two  (2)  per  cent  from  that  specified  may  be  cause  for 
rejection.  For  the  .said  sheared  plates  the  permissible  excess 
variation  shall  run  from  four  (4)  per  cent  for  plates  five 
eighths  (I)  of  an  inch  or  more  in  thickness  to  eight  (8)  per 
cent  for  plates  three  eighths  (f)  of  an  inch  or  less  in  tliick- 
ne.s.s,  the  variations  for  intermediate  thicknesses  being  directly 
interpolated. 


bV.   PONTIBUS. 


Should  Ihf  shipping  weiglit  of  any  eiitiru  oidcr  exceed  hy 
more  timii  one  (1)  per  cent  the  weiglit  couipuled  from  liie 
approved  siiop  drawings,  tlie  amount  in  excess  of  tlic  said  one 
(1)  per  cent  will  not  lie  paid  for,  unless  in  the  entire  order 
tlie  weight  of  plates  exceeding  thirty  six  (36)  inclieH  wide  bo 
greater  than  thirty  (30)  per  cent  of  the  whole,  in  wliicli  case 
the  allowable  variation  shall  be  increased  to  two  (!i)  per  cent. 

wiuniGiii   inoN. 

All  wrought  ircui,  if  any  be  used,  must  be  of  tbc  best 
quidily  obtainable,  tough,  ductile,  librous,  and  of  a  ludforni 
quality;  also  straight,  smooth,  and  free  from  cinder-pockets  or 
injurious  Haws,  buckles,  blisters,  or  cracks.  No  steel  scrap 
shall  be  used  in  its  miinufacture. 

The  tensile  strength,  determined  fron\  test-pieces  in  the 
same  nuinner  as  spe(;ilied  for  steel,  shall  not  fall  below  fifty 
thousand  (50,00(1)  pounds  per  stjuare  inch;  and  the  elastic 
limit  shall  not  be  less  than  twenty-six  thousand  (20,000) 
pounds  j)er  square  inch.  The  alongalion,  determined  in  tlif 
same  nuinner  as  specified  for  steel,  shall  not  be  less  tlian 
twenty  (20)  per  cent. 

All  wrought  iron  must  bend  cold  oi;e  hundred  and  eiglily 
(180)  degrees,  without  sign  of  fracture,  to  a  curve  the  inner 
radius  of  whicli  equals  the  tiiickness  of  the  piece  tested.  Soft 
steel  is  to  be  used  instead  of  wrought  iron  wherever  piac- 
ticable. 


CAHT   IHON. 

Except  where  chilled  iron  is  specified,  all  ciastings  shall  be 
of  tough,  gray  iron,  free  from  injurious  cold-shuts  or  blow- 
holes, true  to  pattern,  and  of  a  workmanlike  finish.  Sample- 
pieces  one  (1)  inch  square,  cast  from  tlie  same  heat  of  metal  in 
sand-moulds,  sliall  be  capable  of  nuiintaining  on  a  clear  span 
of  four  (4)  feet  six  (6)  inches  a  central  load  of  five  hundred 
(500)  pouiuls  when  tested  in  the  rough  bar.  All  castings 
shall  be  straight  and  out  of  wind,  with  proper  and  approved 
uniform  thickness  of  melal,   ami  shall   liave  perfect,  sharp, 


SPKfJIFlCATIONS    FOK   HTKIWi    IN    HIllIMJKS,     K'IC.    -ly.] 

and  vAeau  lines,  angles,  ami  muuldings,  till  re-entrant  angles 
being  properly  filleted. 

CAST   HTKKL 

All  steel  fastings  slinll  he  made  of  acid  open  hcarlli  steel 
containing  from  twenty  tive  inindicdtlis  (O.'j,"))  to  four  tenths 
(0.4)  percent  of  carbon,  and  not  more  than  Die  following  per- 
centages of  other  ingredients : 

Phosphorus,  five  hnndredihs  (0.05). 
Sul[)hiir,  live  hundredths  (0  05), 
Manganese,   eight  tenths  (0.8). 

The  ultimate  tensile  strength  .shall  run  from  sixty  five  thou- 
sand (65,000)  to  .seventy-live  thousand  (75,000)  pounds  per 
square  inch;  tin;  elastic  limit  shall  not  be  less  than  one  half  (i) 
of  the  ultimate  stiength;  and  the  elongation  of  tc^t  specimens 
in  two  (3)  inches  shall  not  be  less  than  tifteeii  (15)  per  cent 
for  rtxtd  castings  or  seventeen  (17)  per  cent  for  movable 
castings. 

All  steel  castings  shall  be  carefully  and  uniformly  annealed, 
and  shall  be  true  to  drawings,  smooth,  clean,  and  free  from 
blowholes,  spongine.ss,  and  all  other  defects.  All  corners 
therein  shall  be  properly  filleted. 

TKSTH  OF    KOLLKR8   FOK   DHAW-8PAN8, 

The  Contractor  shall  make,  at  his  own  expense,  under  the 
direction  of  the  Engineer  or  his  duly  authorized  representa- 
tive, for  each  draw-span,  tests,  not  exceeding  thieu  (:!)  in 
number,  of  full-sized  cast  rollers;  also  any  tests  of  s]>ociinens 
of  the  metal  for  the  same  that  may  be  considered  necessary  by 
the  Engineer  to  determine  its  quality. 


OTHER   TESTS  OF   FULL-SIZE   MEMBERS   OR   DETAILS. 

The  Contractor  shall  make,  at  his  own  expen.se,  tmder  the 
direction  of  the  Engineer  or  his  Insjieclor,  such  oilier  tests  of 
full-size  members  or  details  as  the  Engineer  may  prescribe, 


254 


1)K    I'ONTJHCS. 


provided  tliat  llic  siiid  inumbeis  or  details  are  sliniliir  to  those 
used  oil  the  work,  mid  provided  lliul  the  toliil  cosi  lo  tlie('oii- 
tnictor  of  such  extra  tests  docs  not  exceed  one  (pmrter  of  one 
per  cent  (O.'i.W)  of  the  total  conlract  price  of  the  work. 


WOllKMANWlllI'. 

All  metal  shall  be  carefully  straightened  before  being  turned 
over  lo  the  shops. 

All  workn)aushi|)  shall  be  tirst-class  in  every  particular,  and 
all  portions  of  metal-work  exposed  to  view  shall  be  iieally 
Ihiished. 

All  idle  corners  of  plates  and  angles,  such  for  instance  as  the 
cuds  of  the  unconnected  legs  of  angle  lacing,  shall  be  utally 
cliaiiifercd  olf  at  an  angle  of  about  forly-tive  (45)  degrees,  so 
as  U)  give  a  siglitly  linisii  to  the  work  and  to  avoid  beniling  of 
s  lid  corners  during  shipment  and  erection. 

As  far  as  practicable,  all  parts  siiall  be  so  constructed  as  to 
be  accessible  for  inspection  and  paiuling. 

All  punched  work  shall  be  so  accurately  done  th'U,  after  the 
various  component  pieces  are  assembled  and  before  the  ream- 
ing is  commenced,  forty  (40)  per  cent  <>f  the  lioles  can  be 
entereil  easily  by  a  rod  of  a  diameter  one  sixteenth  (j'j.)  of  an 
inch  less  than  that  of  the  punched  holes;  eighty  (80)  per  cent 
by  a  roti  of  a  diameter  one  eighth  (J)  of  an  inch  less  thsin 
same;  and  one  liuii;lietl  (100)  per  cent  by  a  rod  of  a  diameter 
one  quarter  (|)  of  an  inch  less  than  same.  Any  shopwork  not 
coining  up  to  this  requireinent  will  be  subject  to  rejection  by 
the  inspector. 

SHEAUED   EDGES. 

All  sheared  and  hot-cut  edges  shall  have  not  less  than  one 
(piaiter  (J)  inch  of  metJil  removed  by  pinning  to  a  smooih, 
linished  surface.  Lacing-l)  irs,  fillers,  stay-plates,  and  stringer- 
bracing  connecting  plates  only  will  be  exempt  from  this  re- 
quirement. 

RE-ENTUANT  COllNERS. 

No  sharp  or  xinfi!let{!d  reentrant  corners  will  be  allowed 
anywhere  in  the  work. 


SPECIFICATIONS   FOR   STEEL    IN    UKIDOES,    ETC.    255 


ANNRAMNO. 

Ill  all  cases  where  a  steel  |)iece  in  which  the  full  strength  is 
rwjuired  has  been  partiully  heated  or  bent,  the  whole  piece 
must  be  subsequeiitl}'  annealed.  In  pieces  of  .secondary  im- 
portance, where  the  bending  is  slight,  said  bending  is  to  be 
made  cold,  and  no  annealing  in  such  cases  will  be  required. 
Crimped  web  stiff eners  will  not  reciuire  annealing. 

All  eye-bars  shall  be  carefully  and  uniformly  annealed  at  a 
dark  orange  heat. 

UIVKTS. 

Rivets  when  driven  must  completely  t^.ll  the  holes,  Lave  full 
heads  concentric  with  the  rivet-holes,  and  be  machine-driv  jn 
whenever  practicable.  The  machine  must  be  capable  of  re- 
taining the  applied  i)re8sure  after  the  upsetting  is  completed. 

The  rivet-heads  must  be  full  and  neatly  rinished,  of  approved 
hemispherical  shape,  in  full  contact  with  the  surface,  or  be 
countersunk  when  so  reipiired,  and  of  a  uniform  size  for  the 
same-sized  rivets  throughout  the  work;  and  they  must  pinch 
the  connected  pieces  thoroughly  together.  Flattened  heads 
may  be  used  in  certain  places,  if  necessary  for  clearance. 
Except  where  shown  otherwise  on  the  drawings,  all  rivet 
diameters  are  to  be  seven  eighths  (J)  of  an  inch.  No  loose  or 
imperfect  rivets  will  be  allowed  to  remain  in  any  part  of  the 
metal-work. 

RIVET-HOLES. 

Rivet-holes  must  be  accurately  spaced;  the  use  of  drift-pins 
will  be  allowed  only  for  bringing  together  the  several  parts 
forming  a  member,  and  they  must  not  be  driven  with  such 
force  as  to  distort  the  metal  about  the  holes.  The  distance 
between  the  edge  of  any  piece  and  the  centre  of  a  rivet-hole 
must  never  bo  less  tiian  one  and  a  half  (]«)  inches,  excepting 
for  lattice  bars,  small  angles,  and  wlu.re  especially  shown 
otherwise  on  the  Engineer's  drawings;  and,  wherever  practica- 
ble, this  distance  shall  be  at  least  two  (3)  diameters  of  the 
rivet. 


256 


UK    I'ONTIBUS, 


rUNCniNO   AND   TIEAMINO, 

All  rivet-holes  in  steel-wo) ,.,  if  puncbcd,  sliall  be  made  with 
a  punch  oue-eighth  <l^)  iwAi  in  diametor  less  than  the  diaiiKilur 
of  the  rivet  intended  to  be  used,  and  sliall  be  reamed  to  a 
diameter  oue-si\teenlli  (^ij)  incii  greater  than  that  of  tlie  said 
rivet. 

Before  this  reaming  lakes  i)lace  all  tlie  pieces  to  be  riveted 
together  sliall  be  assembled  and  bolted  into  position,  then  the 
reaming  shall  be  done;  for  one  of  the  principal  ol)jects  of  this 
clause  in  reh  ioi  to  siibpunching  is  to  ensure  the  correct 
matching  of  rivet-holes,  and  the  avoidance  of  holes  of  exces- 
sive diameter.  Haid  clau.se  also  ensures  the  removal  of  most, 
if  not  all,  incipient  cracks  .started  by  the  process  of  punching. 

All  reaming  is  to  be  done  by  means  of  tv'st-drills,  the  u.se 
of  tapered  reamers  being  proidbited,  except  wiiere  twist- 
reamers  cannot  be  employed.  All  holes  must  be  at  right 
angles  to  surface  of  member,  and  all  sliarp  or  rai.sed  edges  of 
iioles  under  lieads  must  be  s'ightly  rounded  off  before  the 
rivets  are  driven. 

All  holes  for  field-rivets,  axcepting  those  for  lateral  and 
sway-bracing,  when  not  drilled  to  an  iron  template,  shall  be 
reamed  while  tlie  connecting  parts  are  tLUiporarily  assembled. 

Pui'.clung  shall  not  be  permitted  in  any  piece  in  winch  the 
thickness  of  the  metal  exceeds  the  diameter  of  the  cold  rivet 
that  is  to  be  used;  but  all  such  pieces  shall  be  drilled  solid. 


BUILT   MKMHEKS. 

Built  members  must,  when  linished,  Ije  true  and  free  from 
twists,  kinks,  buckles,  or  open  joints  between  the  component 
pieces. 

All  abu;ting  surfaces  of  compression-members,  except 
rtinges  of  plate-glnlers  where  the  joints  are  fully  spliced, 
must  be  pLned  or  turned  to  even  bearings  so  that  they  shall 
be  in  as  perfect  contact  througiiout  as  can  be  obtained  by  such 
means  ;  and  all  such  finished  surfaces  must  be  protected  by 
white  lead  and  tallow  before  shipnient  from  the  shop. 


SFKCIFKJATION.S   FOU   STEEL    IN    BlllDCJES,    KVC.   25T 

The  ends  of  all  webs  jukI  chord  or  llaiigo  angles  that  abut 
against  other  wel)s  umst  l>e  fai  c  I  true  and  sciwaro  or  to  exact 
bevel  ;  and  the  end-stifT<.'ncrs  must  l)e  placed  perfectly  tlusii 
with  these  planed  ends,  so  ns  to  tif'ord  a  i)roper  bearing. 
Filling-[)lates  beneatli  end-stilTi'ning  angles  must  be  practically 
flush  with  said  angles,  and  must  m  no  case  project  outside  of 
same  at  tiie  bearings.  If  a  good  and  satisfactory  jol>  of  woik 
cannot  be  obtained  l)y  tins  nieihod,  the  end-stitl'ening  angles 
shall  be  made  one  eighth  (J)  of  an  inch  thicker,  and  the  entire 
ends  sliall  be  planed  after  the  slilTening  angles  are  riveted  on. 

No  web-plate  will  be  allowed  to  pioject  beycjnd  the  flange 
angles,  or  to  recede  more  than  one  eighth  (^)  of  an  inch  from 
faces  of  same. 

All  filling  and  splice  i)lates  in  riveted  work  must  fit  at  their 
ends  to  the  flanges  sunicienlly  close  to  be  .sealed,  when 
painted,  against  the  adinissim  of  water;  bit  they  need  not 
be  tool-finished,  unless  so  specially  indicated  cither  i>n  the 
drawings  or  in  the  si)ecitications. 

EVE-BAUS. 

Except  in  the  case  of  loop-eyes,  no  weld  will  be  allowed  in 
ihc  bf>dy  of  the  eye-bars.  The  heads  of  the  eye-l)ars  sindl  be 
made  by  upsetting,  rolling,  or  forging  into  shape.  A  variation 
from  the  speci/ied  dimensions  of  ihe  beads  will  be  allowed,  in 
thickness  of  one  thirty-second  (.,'.,)  of  an  incii  below  and  oiw 
sixteenth  {f\)  of  an  inch  above  that  specified,  and  in  diameter 
of  one  fourth  (\)  of  an  inch  in  cither  direction.  Eye-bars 
must  be  perfectly  straight  before  Ixu-ing. 

Loop-eyes  shall  invariably  bv'  made  of  wrouglit  iron,  as 
steel  cannot  be  relied  upon  to  afford  a  proper  weld. 


hv 


riNIIOLKS. 

All  pinholes  must  be  bored  tiidy  parallel  and  at  right 
angles  to  the  a.xe«  of  tlie  nvembers,  urdcss  oliierwis<'  >iiown  on 
the  drawings;  and,  in  piece-;  not  adjustable  for  length,  no 
variation  of  more  than  one  tiiirly-second  (;jV'<'f  ""  'iK'h  will 
be  allowei]  in  the  length  between  centres  of  pinholes. 


rioH 


1)H    I'ONTinUS. 


Pinholes  in  e^e-bars  imist  be  in  the  centre  of  the  heads, 
and  on  the  centre  line  of  tlio  bars. 

Bars,  whicli  are  to  be  i)lace(i  side  by  side  in  the  structure, 
shall  be  bored  at  the  same  temperature,  and  shall  be  of  such 
e(iual  length  thai,  when  placed  in  a  [tile,  the  i)in  at  each  end 
will  pass  throngii  the  iioles  at  the  same  time  without  forcing, 

PINS. 

AH  plus  shall  be  turned  accurately  to  a  gauge,  and  shall  l>e 
linished  perfectly  round,  smootli,  and  straight.  All  pins  up 
to  three  and  one-half  ['.]h)  Inches  in  diameter  shall  fit  tiie  pin- 
holes within  one  liflieth  (./„)  of  an  inch,  and  all  i)ins  over 
three  and  one-half  (;{A)  inches  in  diametcH"  shall  lit  their  holes 
within  one  thirty-second  (.j'j)  of  an  inch. 

The  (.'ontractor  must  provide  steel  pilot  imls  for  all  pins  to 
preserve  the  tineads  while  said  pins  are  being  driven. 

TUUNED   BOLTS. 

When  members  are  connected  by  bolts  which  transmit 
shearing  stresses,  the  holes  must  be  reamed  i)urallel,  and  the 
bolts  mubl  be  turned  to  a  driving  tit 


TUUNBlICKLliS,    Nl  TH,    TlIItKADS,    ANU    WASHKUS. 

All  sleeve-nuts, turnbuckles,  and  clevises  must  lie  made  so 
strong  and  still  that  they  will  be  at>le  to  resist  withou;, 
rupture  the  ultimate  pull  of  the  bars  Avhich  they  connect,  and 
without  distortion  the  greatest  twisting  force  to  which  they 
could  ever  t)e  subjected.  They  must  be  made  so  that  the 
tiircailcd  lengths  of  IJie  rods  (Migagcd  can  be  verified. 

The  dimensions  of  all  .square  and  hexagonal  nuts,  except 
those  on  the  ends  of  pins,  shall  be  such  as  to  develop  the  full 
strength  of  the  })oily  of  the  adjustable  member.  No  round- 
headed  bolts  will  \h',  allowed. 

Washers  and  nuls  luust  iiave  uniform  bearing. 

Cast  or  wrought  iron  wasliers  must  i)e  used  under  the  heads 
of  all  timber  bolts  when  the  bearing  is  on  tiie  wood. 


si'KcincA  TioNs   I'oit  ,sri;i:L   in    i;i;ii)(ii:s,   ktc.  ^i.M) 

All  ihrciids,  (!.\(:(;[)t,  Iliose  0:1  llic  oinis  of  pins,  iiiiist  be  of 
the  Uuitt'tl  Sliiles  staiidiinl.  Eacli  udjusling  nut  must  be 
provided  with  lui  effect ive  nut-lock  or  check-waslier, 

HOI.l.KUS. 

Hollers  '-hall  ]h;  turned  iiccurately  to  a  gauire,  and  n)ust 
be  linislied  in'ifecljy  round,  and  to  tlie  correct  diameter  or 
diameters,  fidrn  end  to  end.  Tiie  tongues  and  grooves  in 
I)lates  and  rollers  must  til  snugly,  so  as  to  prevent  lateral 
motion.     Roller  beds  must  be  i)bined. 

ANCtlOK-BOI/rS. 

All  bed-plates  and  bearings  must  be  fox-bolted  to  the 
masonry  or  attached  to  concrete  by  anchor-plates.  The  Con- 
tractor must  furnish  all  bolts,  drill  all  holes,  and  se!  the  bolts 
to  i)lace  with  Portland-cement  grouting. 

All  anchor-bolts  are  to  be  of  soft  steel  witli  cold-pressed 
threads;  and  the  threaded  portion  of  all  such  l)olts  tested  to 
destruction  shall  develop  a  greater  s.iength  than  tliat  of  the 
unthreadc.'d  poi'tion  of  same.  Tiie  lengths  of  t'le  nuts  for  all 
a<ijustable  rods  must  .always  be  great  enough  to  develop  the 
full  .strength  of  the  rod. 

All  anchor-bolts  are  to  be  thoroughly  oiled  but  not  painted 
before  shipment;  and  the  exposed  portions  theri.'of,  after 
erection,  are  to  receive  two  coats  of  paini  at  the  same  lime  the 
rest  of  the  ni'-tal-work  receives  its  two  coats. 

NaMK  PLATKP. 

A  name-plate  of  neat  design  and  llni-^li,  giving  the  mime  of 
the  Contractor  and  the  date  of  erection,  siiiill  be  tii  inly  attached 
to  each  eml  of  every  tlirough-bridge,  and  to  some  prominent 
place  or  places  in  (ill  other  structures. 


■ads 


PAINTINO, 


.\11  metal-work  before  leaving  the  shop  shall  bo  thoioughly 
clc'in-cd  fiotn  all  loose  scale,  rust,  ;i[jd  dirl,  and  shall  tiicjj  be 


360 


I)H    I'ONTinUS. 


given  one  coiit  of  tlie  liest  carbon  primer,  Eiirekji  piiint,  or  :uiy 
otlier  priining-coiit  required  hy  tlie  Engineer,  wliich  coat  shall 
be  thoroughly  dried  l)efore  the  nietiil-work  is  lo  uied  for  ship- 
ment. It  is  absolulely  essential  tliat  the  entire  surface  of  llie 
metal-work  be  thoroughly  cleansed  by  the  most  elTeelive 
known  methods,  such  as  the  use  of  wire-brushes,  then  the 
l)ainter's  torch,  and  in  certain  cases  the  application  of  a  strong 
caustic  solution,  followed  by  sciaping,  washing  with  clean 
water,  and  drying. 

In  riveted  work  all  surfaces  coming  in  contact  shall  be 
extra  well  painted  before  being  riveted  together,  liottonis  of 
l)edplates,  bearing-plates,  and  any  other  parts  which  are  not 
accessible  for  painting  alter  erection  snail  have  three  coats  of 
paint,  one  at  the  sl>o[),  the  other  two  in  the  field,  before 
erection.  Pins,  bored  pinholes,  turned  friction-rollers,  and 
all  other  polislied  surfaces  shall  be  coated  with  white  lead  and 
tallow  before  shipment  from  the  shop. 

After  the  structure  is  ere<;ted  the  metal-work  shall  be 
thorougbly  (;Uansed  from  mud,  grease,  or  any  other  objection- 
able material  that  may  be  found  thereon,  then  thoroughly  and 
evenly  painted  v.itli  two  (2)  coats  of  paint  of  any  kind  that 
the  Engineer  may  adopt. 

All  tiircvf  coats  of  paint  given  \o  the  metal-work  are  to  be  of 
distinctly  dilTen  nt  shades  or  colors  ;  and  the  second  coat  must 
be  allowed  lo  dry  thorougiily  befo.e  the  third  coat  Is  ajjplied. 

No  thinning  of  paint  with  turpentine,  benzine,  or  other 
thinner  will  be  allowed  without  special  written  iiermission 
from  the  Engineer 

No  painting  is  to  be  done  in  wet  or  freezing  weather. 

All  painting  is  to  be  done  in  iv  thorougli  and  workmaidik 
maimer,   to   the  satisfaction    of    the    Engineer,  and  no  paint 
whatever  is  to  be  used  on  the  .structure  without  lirst  being 
approved  by  the  Engineer. 

All  the  mafrrials  fur  painting  shall  be  subject  at  all  times  (o 
the  closest  inspection  and  chemical  analysis;  and  the  detection 
of  any  inferior  (juality  of  such  material,  in  either  siiop  or 
field,  shall  involve  the  rejection  of  ail  such  suspected  material 
;it  hand  and  the  scraping  and  repainting  of  those  portions  of 


SPKClFlCATlONS   I'Oll   SIKKL   IN    HKIDGES,    ETC.    201 

tlio  work  whicli,  in   the  opinion   of  tlio   Eiigiiiecr,   wen;  de- 
fectively painted  on  uc  oiint  of  such  inferior  niateriid. 

All  recesses  which  would  retain  water  or  through  which 
water  could  enter  must  be  lilled  with  thick  jiainl  or  some 
water-proof  cement  before  receiving  final  painting.  All  sur- 
faces so  close  together  as  to  |Mcveiit  tiie  iiiseiliou  of  paint- 
brushes must  be  painted  thoroughly  by  using  a  piece  of  cloth 
instead  of  the  brush. 

SHIPPING. 

All  parts  shall  be  loaded  carefully  so  as  to  avoid  injury  in 
transportaiion,  and  shall  be  at  the  Coiitru(;l(jr's  risk  until 
erected  and  accepted. 

In  shipping  hjiig  plate  girders  great  care  is  to  be  talien  to 
distribute  the  weight  properly  over  i1k;  two  cars  that  support 
them,  and  to  provide  means  for  peiinitliiig  tlie  cars  to  pass 
around  curves  without  disturl)iiig  the  loading.  In  both  the 
handling  and  shipHient  of  metal-work  every  care  is  to  be 
taken  to  avoid  bending  or  straining  tlie  pieces  or  damaging 
the  paint.  All  pieces  bent  or  otherwise  injured  will  be  re- 
jected. 

TIMBEK. 


All  timber  must  1)e  of  the  best  (pinlity,  sawed  true  and  out 
of  wind,  full  size,  and  free  from  wind-shakes,  large  or  loose 
knots,  decayed  wood,  sap,  worm-holes,  or  any  other  defect 
that  would  impair  its  strength  or  durability. 


ERECTION. 

The  Contractor  shall  furnish  all  staging  and  falsework,  and 
shall  erect,  adjust,  and  p.aint  all  of  the  metal-work  ready  for 
the  timber-floor.  He  shall  also  furnish  and  lay  the  latter  and 
put  on  tlie  track-rails,  unless  there  be  a  written  agreement  to 
the  contrary. 

The  Contractor  shall  employ  suitable  mechanics  for  every 
kind  of  mechanical  work,  and  shall,  at  the  recjuest  of  the  eu- 


2G3 


I)K    I'OXTIHUS. 


giiiet;r,  dischiirge  any  workman  whom  the  said  Engineer  shall 
deem  incoinpelent.  negligent,  or  untrustworthy. 

All  nmterial  of  whatever  kind  siiall  he  suhjeet  to  inspection 
and  approval  at  any  time  during  the  progress  and  until  the 
final  completion  of  the  work  ;  and  tlie  entire  work  shall  l)e 
conslnieted  in  a  siihstanlial  and  workmanlike  manner,  anil  to 
the  satisfaction  and  acceptance  of  the  Engineer. 

DKKKCTIVE    WOHK. 

The  Contractor,  upon  being  so  directed  by  the  Engineer, 
shall  remove,  rebuild,  or  make  good,  witiiout  charge,  any 
work  wliich  the  said  Engineer  may  consider  to  be  defectively 
executed.  The  fact  tiial  any  defective  material  in  the  struc- 
ture had  been  previously  accepted  l)y  the  oversight  of  the 
Company's  engineers  or  inspectors  shall  not  be  considered  a 
valid  reason  for  the  (Contractor's  refusing  to  remove  it  or  make 
it  good.  And  uiu.il  such  defective  work  is  removed  and 
made  good,  tiie  Engineer  shall  deduct  fif»m  the  jiartial  pay- 
ments or  th(!  final  payment,  as  the  case  may  Ix-,  whatever  sum 
for  siKiii  defective  work  as  may,  in  liis  opinion,  appear  just 
and  equitable. 

niKKCTIONM  TO  CO.NTK.VCTOH. 

Tn  case  that  the  (-"ontractor  shall  not  la;  present  upon  the 
work  at  any  lime  when  it  may  la;  ni'ccssar}'  foi  the;  Engineer 
to  give  instructions,  the  foreman  in  <  liari^c  for  the  lime 
being  sinvll  receive  and  obey  any  orders  liiat  tlie  Engineer 
nniy  give. 

The  Contractor  slnill  commence  work  at  .sucli  points  as  tin,' 
Engineer  may  direct,  and  shall  conform  (o  his  ilir'ctioni  a^io 
tlie  Older  and  lime  in  widch  Mie  diirerent  piirls  of  tiie  work 
shall  br  done,  as  well  n.s  to  the  force  requireil  to  coritplete  the 
work  Ht  llie  date  spccifled. 


CI  USING   THOUOfi.M FAKES. 


The  Contractor  and   his  employees  shall  so  conduct  their 
operations  as  not  to  clo.so  any  thoroughfare  by  land  oi   water 


SPECIFICATIONS    lOll    liltKCrnON  OV  UUIDGES,  KTC.    203 


wilhoul  the  wiiituii  (roiiseiit  of  the  proper  aulliorities  of  such 
thorough  fare. 

KKSI'ONSIBIMTY   FOR   ACCIDENTS. 

The  Contractor  sliall  assimu'  and  be  responsible  for  all  acci- 
dents to  men,  animals,  and  materials  before  the  acceptance  of 
the  structure  ;  and  must  remove  at  his  own  ex])ense  all  false, 
work,  rubbish,  or  other  useless  material  caused  hy  liis  oper- 
ations; and  such  work  sliull  be  included  ms  a  part  of  the  work 
to  be  performed. 

The  Cctnlractor  shall  place  sutlicienl  and  proper  guards  for 
the  prevention  of  accidents,  and  shall  put  up  and  maintain  at 
night  suitable  and  sullicient  lights. 

DAMAOES. 

The  Contractor  shall  indemnify  and  save  harmless  tlie  Com- 
pany against  all  claims  and  demands  of  al!  parlies  whatsoever 
for  damages  or  compensation  for  injuries  arising  from  any 
obstructions  (Mealed  by  the  Contractor  or  his  er.iployees,  or 
from  any  neglect  or  omission  to  provide  proper  lights  and 
signals  during  the  construction  of  the  work. 

ALTERATION    OF   l'I..\NS. 

The  Engineer  shall  have  the  power  to  vary,  extend,  in- 
crease, or  dimiinsh  the  ([uantity  of  the  work,  or  to  dispense 
with  a  portion  thereof  during  its  pn)gr(!ss  without  impairing 
the  (KMliact;  and  no  allownnce  will  be  made  the  (.\>n  tract  or 
e.Kctpi  for  the  work  aclutiUy  done.  In  case  any  change  in- 
volve the  exe(  ution  of  work  of  a  class  not  herein  provided  for. 
the  (Contractor  shall  jierform  the  sanu;  and  be  paid  the  aoti^l 
cost  tiieniof  phis  the  percentage  for  prolil  agreed  upon  in  the 
contract.  In  this  case  the  Contractor  must  furnisii  the  Engi- 
neer with  satisfactory  vouchers  for  all  labor  and  material 
expemled  on  the  work. 

STKICTNK88  OF   IKSPECTION. 

All  materials  and  workmanshij)  will  be  thoroughly  and 
carefully  inspected,  and    the  Contractor  will   be   lield    at  all 


264 


m:  poxN'TiBUS. 


times  tt)  tho  spirit  of  tlie  sivcificatious;  but  nothing  vvill  be 
done  by  tlio  Conipun^-'s  engineers  or  inspectors  to  give  tin: 
Contnictor  needless  \v(»ny  or  iuino^iinee,  the  intent  of  botli 
spec ilicitt ions  and  inspection  being  simply  to  obtain  for  the 
(Jonipjiny  work  that  will  be  liist-class  in  every  particular  and 
a  credit  to  every  one  connected  with  its  designing  and  con- 
strucliou. 

SPIRIT   OF  THE   SPECIFIC ATTONB. 

The  nature  and  s|)irit  of  Ihe-e  specilicalions  are  to  pro- 
vide for  the  work  herein  enumerated  to  be  fully  completed  in 
every  detail  for  the  purpose  designed;  and  it  is  hereby  under- 
stood thai  tiie  Contractor,  in  acc(!pling  the  contract,  agrees  to 
fmiiish  any  and  every  thing  ne(!css;iry  for  such  construction, 
notwitlistanding  any  omission  in  the  drawings  or  speciticik- 
tious. 

ENOINKEK. 

Whenever  ill  these  specirtcalions  the  term  "Engineer"  is 

employed,  it  is  understood  tlial  it  is  to  mean  the       

Engineer  of  the Company  or  the  duly  author- 
ized representative  of  same. 


TENDERR. 

Tenders  for  all  work,  whenever  it  is  practicable,  shall  be 
made  on  schedule  prices,  lump-sum  bids  being  accepted  only 
for  such  parts  as  .steam  or  electric  machinery,  which  could 
nut  well  be  paid  for  by  the  pound. 

All  Lenders  are  to  be  made  in  strict  accordance  with  ihe 
plan;;  and  specifications  submitted  to  liidders  by  lh(i  Engineer; 
and  no  bids  based  upon  suggested  changes  in  same  will  be 
considered. 

In  awarding  contracts,  pieference  will  be  given  to  those 
bidders  in  whose  shops  the  piece-work  system  is  least  em- 
ployed. 


CHAPTER  XIX. 

TFIE    COMPROMISE  STANDARF>    SYSTEM   OK    LIVE  LOADS  KOR 
RAILWAY  BRIDGES  AND  THE  EyUIVALENTS  FOR  SAME. 


In  1893  the  author  published  a  Hiiiall  paiiiphh't,  now  out  of 
print,  whicli  bore  the  above  title.  Its  contents  are  reproduced 
iiere  instead  of  in  a  second  edition.  The  various  h\v.\)h  talien 
in  its  preparation  were  as  follows  : 

In  1891  tiie  author  presented  to  the  Aniericnn  Societ}'  of 
("ivil  P^ngineers  a  paper  entitled  "Some  Disputed  Points  in 
Railway-Bridge  Desigi.ing,"  in  which  he  advocated  the  adop- 
tion of  a  few  standard  train-loads  for  railroad  bridges,  instead 
of  the  ahnost  innumerable  ones  then  in  u^e,  ollVrcd  a  set  of 
loads  for  discussicn,  and  urged  that  the  "  lupuvalent  rniform- 
Load  Method  '  of  coniputitig  stresses  be  adopted  instead  of 
the  burdensome  method  of  wheel  concentrations  that  liad  been 
in  vogue  for  the  preceding  ten  years.  This  p.-iper  received  a 
very  thorough  discussion,  from  which  it  was  evident  that 
bridge  engineers  and  railroad  engineers,  as  a  whole,  would  be 
glad  to  settle  upon  a  few  standard  loading-*,  and  to  adopt 
some  simple  etpdvalent  method  of  (computing  stresses.  Most 
of  those  who  desired  the  ab;indonmeut  of  the  "  Coiu'entrated 
Wheel-load  Method,"  advocated  tlie  adoption  of  the  "  Ecpiiv- 
alent  Uniform-Load  Method,"  but  a  few  favored  either  the 
"Single"  or  the  "Double  Concentration  Method,"  with  a 
constant  car-load. 

Tliis  paper,  with  the  discu.ssious,  was  published  in  the  Feb- 
ruary and  March  1892  number  of  the  Transnctionn  of  the 
American  So(!iety  of  Civil  Engineers,  and  was  reviewed  very 
generally  by  the  tcchiucal  ))r(>ss,  attention  being  paid  princi- 
pally to  the  subject  of  equivalent  loads.  These  reviews  started 

365 


',*<;(; 


l)K    I'ON'rilJl'S. 


ii  s(!iies  of  It'ltcrs  by  tliciuilliordiid  others,  that  wcic  printed  iit 
first  ill  tlic  lidUroad  (hizette,  and  L.ti-r  iilsu  in  tin;  Kngineeriny 
Record,  in  wliicli  li'ltcrs  the  suliject  of  eciiiivuleiils  was  thor- 
oiigiily  and  cxhiiustively  treated.  These  jiroved  tliat  the 
"  E(iiiivaieiit  L'uiforiiiLoad  Method  "  gives  results  wliicdi  are 
ueeiirate  eii()ut,di  for  idl  practical  jjiirposes,  and  tiiat  iieiliier 
the  "Single  Concent  liiled-Load  Metliod  "  nor  \\\v  "Double 
Concent rated-IiOad  Method  "  gives  results  coinciding  at  all 
closely  with  those  found  by  the  theoretically  exact  method  of 
"  Wheel  Cniicentiatioiis." 

Ill  November  1892  the  author  sent  a  circular  letter  to  all 
the  chief  engineers  of  railroads  in  the  United  States  and  Can- 
ada who  were  members  (in  any  grade)  of  the  American  Socii'iy 
of  Civil  Engineers,  and  to  every  other  memlHjr  of  thai  society 
connected  wilh  or  specially  interested  in  tht;  designing,  build- 
ing, or  operating  of  railroad  bridges.  'I'lds  letter  soli(;ited  a 
ballot  on  certain  "  Disputed  Points  in  Hallway  IJiidge  Design- 
ing." foremost  among  which  were  tiioscjof  standard  live  loads 
and  a  siin|)le  ecpiivalent  method  for  (•omputalion.  Tin;  num- 
ber of  responses  received  was  as  great  as  could  liave  been  (  \- 
pected  ;  and  thu  result  was  that  about  eighty-two  per  cent  of 
those  who  voted  favored  and  eighteen  per  c(;iit  opposed  the 
adoption  of  "  a  iStandard  System  of  Live  Loads  for  Railway 
Bridges  "  similar  to  that  proposed  by  the  author.  Eighty-two 
per  cent  also  of  those  who  voted  were  in  favor  of  abandoning 
the  "  Concentrated  Wheel-Load  Method,"  and  eighteiii  per 
cent  were  in  favor  (»f  retaining  it.  Of  lh<!  foruKM',  seventy- 
eight  per  cent  favored  the  "Equivalent  L'ldrorin-Load 
Method,"  and  twenty-two  per  cent  were  in  favor  of  either 
the  "Single"  or  the  "Double  Con<'enlration  Method,"  A 
number  of  gentlemen  who  resi)onded  made  valuable  sugges- 
tions in  respect  to  the  standard  system  of  live  loads  i)roponnd- 
ed,  and  by  the  aid  of  these  the  author  i)reparcd  a  i)roposed 
"Compromise  Standard  System  of  Live  Loads  for  Railway 
Bridges,"  and  submitted  the  same,  as  bL-fore,  for  final  l)allot 
in  May  1898. 

The  number  of  replies  received  showed  that  great  interest 
was  t-aken  in  the  (piestion  ;  and  the  result  of  the  ballot  was 


('OMPKOMISK    SIANDAIM)    SVsTlvM    Ol'    1,1  VK    LOADS.    t.'()T 

iiiiH'ly  per  cent  in    favor  and  ten  per  cent  opposed  to  the  pro- 
posed stHndanl. 

Next  the  iKuiiplilet,  was  piililislicd  and  distril)iited  (piite  pvn- 
eially  among  tliose  enirincers  interested  in  tlie  suiiject  of 
bridges,  a  copy  heing  sent  not  only  to  every  one  wlio  iiad  re- 
plied to  tiie  ballots,  but  alsct  to  every  railroad  chief  engineer 
in  the  United  States,  Canada,  and  Mexico  whose  address  was 
given  ill  Poor's  Miiiiual.  To  these  ciiief  engineers  there  was 
also  seut  another  circular  letter  with  a  ballot  that  read  as  fol- 
lows : 

^  T)  )  N  u '  Wree  ^^  "'^''  ^''^  "  Compromi-e  Standard  System 
of  Live  Loads  for  Railway  Biidgcs"  when  calling  for  bids  on 
railroad- bridge  work,  or  when  having  plans  prepaied  for  rail- 
load  bridjres. 


I 


Agree 
I)i)  Not  Agree 


to  specify  tluit  the  "  Equivalent  Uniform- 


Loa<l    ISIelhod"  is  to  be  used  in    computing  slres-es  iu  the 
bridges  that  are  to  be  designed  for  my  road. 
Signature  of  Voter. 


Chief  Engineer  of  the 


Over  one  Inindred  chief  engineers  thus  addressed  voted  iu 
favor  of  botli  propositions,  and  very  few  were  ()|ipostil. 

The  pamphlet  has  now  been  in  use  more  than  foiu"  years, 
and  has  ' n  ti  in  such  demand  that  the  first  edition  (a  huge 
one)  hn-^bein  iwhausted.  All  those  who  have  used  its  nielhods 
indorse  ■Hiirii  7  both  the  loads  specified  and  the  Equivalent 
Uni'orm  r,i);t,  l.  Method. 


MKTHOD   OF    UTIUZINQ  TIIR    KQUIVAI.ENT   I,OAD8. 

Tn  calling  for  bids  on  bridge  work  to  l)e  accomiianied  with 
designs  for  the  structures,  a  railroad  engineer  can  nominate 
any  bridge  sjiecilicatioiis  whatsoever,  standard  or  otherwise, 
and  at  the  same  time  specify  that   the  live  lo;ids  are  to  be 


^ 

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IMAGE.  EVALUATION 
TEST  TARGET  (MT-3) 


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Photographic 

Sdences 

Corporation 


23  WEST  MAIN  STREET 

WEBSTER,  N.Y.  14580 

(716)  872-4503 


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'2(iS 


DK    POVTIBUS. 


taken  from  the  "Compromise  Standard  System,"  and  that  tbe 
"Equivalent  Loads"  thereof  are  to  be  employed. 

In  this  "System"  will  lie  found  from  "  Class  Z"  to  "Class 
T,"  inclusive,  a  close  approximation  to  any  live  load  that  an 
engineer  is  likely  to  want  to  use  ;  and  if,  for  a  certain  c:ir- 
load,  some  engineer  should  prefer  a  heavier  or  ligi^ler  engine- 
loading,  I'O  can  obtain  practically  what  he  wislies  by  speci- 
fying that  one  class  is  to  be  used  for  floor  systems  and  primary- 
truss  members,  and  another  class  for  main-truss  members. 
The  author  does  not  advise  this,  however,  except  in  the  case 
of  double-track  bridges,  where  it  would  be  advantageous  to 
use  a  certain  class  for  floor  systems  and  primary-truss  mem- 
bers, and  a  lighter  class  for  the  trus.ses,  because  the  chanct-s 
of  there  being  two,  full,  maximum  train-loads  on  the  span  at 
the  siiMie  time  are  generally  very  small.  It  might  be  well  to 
carry  this  idea  even  further  by  specifying,  for  instance, 
"Class  V  "  for  stringers,  "  Class  W  "  for  floor-beams  and  pri- 
mary-truss members,  and  "Class  X"  for  main-truss  members 
of  doubk'-tiack  bridges.  Such  a  method  would  be  in  accord- 
ance with  the  theory  of  probabilities  ;  but  it  would  not  apply 
to  single-track  bridges,  for  which  the  locomotive  and  car 
loads  of  the  "  Compromi.se  Standard  System "  have  been 
properly  adjusted. 

The  "Equivalent  Uniform-Load  Method"  reduces  to  a 
minimum  (he  labor  of  making  computations  of  stresses  in 
bridges.  The  correctness  of  this  statement  will  be  rendered 
evident  by  the  ensuing  explanations  of  the  use  of  the  method. 
As  for  its  exactness,  if  any-one  has  any  doubt  whatsoever 
about  its  closeness  of  approximation  to  the  theoretically  cor- 
rect method  of  wheel-concentrations,  let  him  read  the  author's 
letter  in  the  Railroad  Oazette  of  July  28,  1893.  An  inspection 
of  Table  I  of  that  communication  shows  that  no  reasonable 
man  can  object  to  the  "Equivalent  Uniform-Load  Method" 
because  of  its  want  of  exaciness. 

In  designing  a  bridge,  one  conimences  naturally  with  the 
stringers,  then  passes  to  the  floor-beams;  and  afterwards  to  the 
trusses  ;  so  let  us  follow  this  order. 


COMPROMISE    STANDARD    SYSTKM    OK    LIVE    LOADS.    2i)9 


STRINOEKS. 

From  Plate  III  find  tlie  equiviilont  live  loiul  per  lineal  foot 
for  a  spun  eqiiul  to  the  panel  length,  add  to  same  the  assumed 
weight  per  foot  of  two  stringers  iind  I  lie  Hoor  they  supporl, 
and  divide  the  sum  hy  two,  calling  the  result  ic  ;  then  find 
the  total  bending  moment  at  mid-spun  hy  substituting  in  the 
well-known  formula, 

where  I  is  the  panel  length  in  feet,  and  M  is  the  recjuired  mo- 
ment in  footpounds. 

Should  the  total  end  shear  be  required,  it  can  be  found  for 
each  stringer  by  adding  together  the  end  shear  given  on  1*1. 
II  and  the  total  weight  of  one  stringer  \n\)x  the  floor  tliiit  il 
carries,  and  dividing  the  sum  by  two. 

FLOOH-HEAMS. 

In  proportioning  a  floor-beam,  tlio  important  thing  to 
ascertain  is  the  total  concentration  at  the  point  where  two 
stringers  meet.  The.  live-load  concentration  is  to  be  found  by 
multiplying  together  the  panel  length  and  the  equivalent  uni- 
form load  per  lineal  foot  given  on  PI.  Ill /or  a  spin  etjiinl  to 
twice  the  panel  length,  and  dividing  the  product  by  two.  It  is 
unnecessary  to  describe  here  how  the  dead-load  concentration 
at  each  stringer  support  is  to  be  found.  Nor  is  it  necessary 
to  do  more  than  merely  mention  that  the  live-load  concentra- 
tion obtained  for  the  floor-beam  is  the  same  as  that  required 
in  finding  stressesln  primary-truss  members. 


TRU88E8. 

These  can  be  divided  into  two  kinds,  viz.,  those  with  equal 
panels  and  parallel  chords,  and  those  in  which  the  panel 
lengths  are  unequal,  or  the  chords  are  not  parallel,  or  both. 
In  the  first  case,  the  stresses  can  be  determined  most  expedi- 
tiously by  substitution  in  tabiduted  formuUp,  and  io  tbc  secom) 
case  by  the  grapliical  method. 


270 


I)E    I'ONTIIU'.S. 


Case  I. 

From  Plate  IV  find  the  equiviileiit  uniform  live  load  per 
lineal  foot  for  the  given  span  iengtli  and  multiply  same  by 
the  panel  length,  calling  the  product  L.  For  siiigh;  trad; 
bridg  s  this  must  be  divided  by  two.  All  the  live  load  stresses 
in  midu-truss  members  of  siugle-iulersection  bridges  can  be 
found  by  substituting  this  value  of  L  in  Table  XVII. 

Just  here  it  is  proper  to  remark  that  the  "  EiiuivaUnI  Uni- 
form-Load Method  "  is  not  applicable  to  trusses  of  multiple 
intersection  ;  but  the  most  approved  modern  practice  in  bridge. 
engiueeriug  does  not  eouutenaiice  the  building'  of  trusses  or 
girders  having  n\ore  tlnin  a  single  system  of  canceli.;ti<)n  Tlie 
"Eq\iivalent  Uniform  Load  Method"  does  howevt  r,  apply 
to  trusses  wiih  divided  panels,  such  as  the  Petit  truss;  Itui  as 
this  style  of  truss  nowadays  involves  almost  inv.'.rialily  a 
polygonal  lop  chord,  its  trea'raent  herein  will  come  under 


Com  II. 

Where  trusses  have  unequal  panels  or  chords  not  parallel, 
the  first  step  to  take  is  the  finding  of  all  the  dead -load  stresses 
by  tlie  graphical  method,  starling  from  one  end  of  the  span 
and  working  towards  the  middle,  where  the  last  stress  is 
checked  by  the  method  of  moments,  and  the  correctness  of 
the  entire  graphical  work  is  thereby  proved. 

The  next  step  is  to  find  from  PI.  IV.,  as  in  Case  1,  the 
equivalent  live-load  per  lineal  foot  for  the  span,  and  there- 
from the  value  of  the  panel-truss  live-h)ad  L.  Next  set  a  slide- 
rule  for  the  ratio  of  dead  load  per  lineal  foot  and  the  equivii- 
lent  live  load  pei-  lineal  foot  for  the  span,  and,  by  referring  to 
the  dead-load  stresses  already  found,  read  oil  from  the  rule  all 
of  the  live-load  stresses  in  chords  and  inclined  end  posts. 

Next  assume  that  there  is  an  upward  reaction  at  one  end  of 
the  span  equal  lo  1,000  pounds,  10.000  pounds,  or  100,000 
pounds  (according  to  the  size  of  the  bridge),  caused  by  a  load 
placed  at  the  first  panel  point  from  the  other  end  of  the  span, 
then  find  graphically  the  stress  in  each  web- mem  her  from  end 
to  end   of  span,  caused   by  this  assumed  upward   reaetiop. 


(JOMl'UOMISK    STAN'DAUI)   SYSTKM    OF    LIVE    LOADS.    ~ 


Tlieii  calculutc  the  value  of  the  live-load  reaction  for  the 
iiiiiximum  stress  in  each  web-member  by  meaus  of  the  slide- 
rule  and  the  following  formula  and  table,  in  which  n  is  the 
number  of  panels  in  the  span,  7i'  is  the  number  of  the  panel 
point  at  the  hend  of  the  train,  coiniting  from  the  loaded  end 

of  the  span,  and  C  is  the  coefficient  of  — . 


n 


Live-load  reaction  for  the  head  of  train  at  n'  =  0  X 


L 
n 


of 


n' 

C 

H' 

C 

n' 
18 

C 

n' 

C 

1 

1 

1 

28 

91 

19 

190 

2 

3 

8 

36 

14 

105 

21) 

210 

3 

6 

9 

45 

15 

120 

1   «' 

231 

4 

10 

10 

55 

16 

136 

!   22 

253 

5 

15 

11 

66 

17 

153 

33 

270 

6 

21 

12 

78 

18 

171 

24 

i 

300 

Then,  still  using  tiie  slide-rule,  find  the  greatest  live-load 
stress  in  each  web-meinber  by  the  following  equation  : 

Stress  required  = 

Actual  Reaction 


Stress  from  Assumed  lleaction  X 


Assumed  Reaction' 


Where  the  panels  are  divided  as  in  the  Petit  truss,  and 
where  inclined  subposts  are  emi)lo3'ed,  the  tensile  stress  in 
the  upper  half  of  each  main  diagonal  thus  found  will  have  to 

be  corrected  by  subtracting  therefrom  a  stress  equal  to  --  sec. 

.1,  where  A  is  the  inclination  of  the  diagoniil  to  the  vertical. 
But  when  inclined  subties  are  used  instead  <  "■  iu(tlined  sub- 
posts,  the  correction  just  referred  to  will  api»ly  only  to  the 
compressive  stresses  in  the  lotoer  halves  of  the  main  diagonals. 
The  reason  for  making  this  correction,  as  will  be  at  once  evi- 
dent to  any  one  who  is  accustomed  to  finding  stresses  in  Petit 
trusses,  is  that  the  metiiod  above  outlined  ignores  the  sub- 
division of  the  panels  ninn  ascertaining  by  graphics  the 
stresses  caused  by  lheas.«uiued  u])ward  reaction. 


272 


DB    P0NTIBU8. 


Tu  comparing  the  equivalent  loads  for  spans  of  one  Imn- 
dred  feet,  ijiven  on  PI.  Ill,  with  those  given  on  PI.  IV,  au 
apparent  discrepancy  will  be  noticed.  This  is  dne  to  the  fact 
that  PI.  Ill  is  for  plate-i-irder  spans,  for  which  the  equivalent 
loads  were  ohtained  fronj  the  bending  moment  at  mid  span; 
while  PI.  IV  is  for  iru.ss  bivas,  for  which  the  e(ivuvaleut 
loadttare  the  average  of  those  at  all  of  the  panel  points. 


hi 


CHAPTER  XX. 


TIMBER   TRESTLES. 


TiMBKR  trestles  can  be  divided  into  two  geueml  classes, 
via  :  Fii'Ht,  Pile-trestles,  or  those  in  which  each  bent  is 
formed  of  several  piles,  a  cup,  and  transvt-rse  sway-bracing  ; 
and, 

Second,  Framed  Trestles,  or  tiiose  in  wliicii  each  bent  is 
composed  of  squared  timbers  framed  togetiier  and  braced. 

Owing  to  the  excessive  length  of  piles  r(qiiire>!  for  greater 
height.'^,  pile-treslles  should  riin-ly,  if  ever,  exceed  thirty  feel 
in  height,  while  framed  trestles,  if  i>r.n>erly  di-s'gneii  for 
rigidity  as  well  as  for  strength,  maybe  carrie  1  up  to  much 
greater  heights,  the  economic  limit  being  probably  about  one 
hundred  feet. 


l'IliE-TUK«TIiK8. 

ITie  Itents  of  a  pile-trestle  should  contain  at  least  four  pilcjs 
each.  Where  tlie  trestle  does  not  exceed  ten  feet  in  height, 
the  piles  may  be  driven  vertically,  and  no  sway-bracing  need 
be  used,  i)rovi(led  that  the  piles  have  a  good  penetration  in 
reliable  material.  For  greater  heights  of  trestle  than  ten  feet, 
the  two  outer  piles  of  each  bent  shcmld  be  given  a  batter  of 
from  two  to  three  inches  (o  the  vertical  foot.  Each  bent 
should  also  be  bmced  with  one  or  two  sets  of  sway-bracing, 
each  composed  of  two  3"  X  10"  yellow-pine  diagonals,  thor- 
oughly bolted  to  the  piles,  wherever  they  cross  them,  by  J" 
bolts.  Wherever  the  piles  are  of  irregular  .sizes,  they  should 
be  trimmed  off  so  as  to  make  the  diagonal  bracing  tit  prop- 
erly. 

The  piles  for  such  bents  should  be  so  spaced  laterally  as  to 

373 


274 


i)E  rovTinus. 


1  I: 


givo  great  I  nu  is  verse  rigidity  to  the  structure,  and  at  the  same 
lime  afford  .•unj>le  support  for  tlie  cnps.  A  good  spacing  is  us 
follows  :  Distance  centre  to  centre  of  outer  piles,  11'  0"  ;  dis- 
tance centre  to  centre  of  two  inner  i>il\is,  4'  6  ". 

The  caps  should  b^j  at  least  VI"  X  14"  X  14',  placed  on  edge 
and  attached  to  the  piles  by  means  of  I"  drift-bolts. 

For  ordinary  pile-trestles  in  fairly  firm  soil  no  longitudinal 
sway-bracing  will  I)e  required  for  heights  below  ten  feet;  but 
for  heights  between  ten  and  twenty-two  feet,  single-deck, 
longitudinal  sway-bracing  should  be  used  in  every  fifth  panel, 
so  as  to  prevent  the  structure  from  moving  longitudinally  as  a 
whole  because  of  thrusl  of  trains.  For  heights  greater  than 
twenty-two  feet,  each  allernate  panel  should  be  braced  longi- 
tudinally by  double-deck  bra<'ing,  so  as  to  hold  the  piles  at 
mid-height,  and  thus  strengthen  them  as  cohnnns;  the  trans- 
verse sway-bracing  for  tliese  cases  should  also  be  double-deck 
for  the  same  reason. 

For  ordinary  pilc-tresthvs  up  to  twenty-two  feet  in  lieight 
tlie  panels  should  be  a  trifie  less  than  fourteen  feet  in  length, 
while  for  greater  heights  either  the  same  length  may  be  used 
or  alternate  panels  may  be  made  from  twenty-four  to  twenty- 
eight  feet  long  by  trussing  tlie  stringers,  according  to  which 
of  the  two  methods  is  the  more  economical. 

The  stringers  under  each  rail  should  be  built  of  three  runs 
of  timber  generally  sixteen  inches  deep,  the  sizes  being  deter- 
mined from  the  loading  and  by  using  an  intensity  of  two 
thousand  pounds  for  the  extreme  fibre,  when  impact  is  in- 
cluded. The  stringer  timbers  are  to  be  separated  from  each 
other  at  tiie  panel  points  by  means  of  timber  packing-blocks, 
which  are  to  serve  also  as  splice-timbers.  These  limber  blocks 
should  be  at  least  three  iiiclies  thick  and  six  feet  ir.  length, 
and  should  have  at  least  four  bolts  through  them.  They  are 
to  be  separated  from  the  stringers  by  small  cast-iron  fillers 
three  quarters  of  an  inch  thick,  so  as  to  prevent  the  timbers 
from  coming  in  direct  contact  with  each  oilier.  The  splice- 
timbers  must  be  made  wide  enough  to  project  an  inch  or  two 
below  the  bottoms  of  stringers,  and  must  be  nolchid  over  the 
caps  sous  to  hold  the  .stringers  firmly  in  j>lace.     The  diatauCf 


TIMHKU  TUKSTLKS. 


215 


from  centra  to  n!Ulre  i)f  middle  stringers  sliould  l)c  live  ffci. 
Iiileriiicdiale  (.'.ist-iruu  sepunilors  witli  l)ult.s  sliuiild  be  ii>ed 
between  ndjiiceut  stringer-timbers,  ut  distuuces  not  tu  exceed 
Jive  feet  centres. 

Tlie  lenntli  of  (he  striager-timljers  for  ordinary  trestles 
sbould  be  iwentyeigirt  feet,  .-o  as  to  extend  over  two  panels, 
iind  thus  .stiffen  i!ii:  floor  system  materially. 

The  ties  should  bo  8"  X  8"  X  10".  They  should  be  dapped 
over  the  stringers  at  least  one-lialf  inch  und  S|)aced  thirteen 
Inches  fronj  centre  to  centre. 

Insi<le  und  ouLside  giiurd-ruils  should  bo  used  j'or  all  tres- 
ilrs.  and  at  each  end  of  every  trestle  .some  satisfactory  kind  of 
reniiliiig  device  should  be  used.  Tiie  outer  guard-rail  should 
be  made  of  a  6"  X  H"  timber  laid  thit  and  dapped  one  inch  on 
the  ties.  The  inner  faces  of  the  outer  guard-rails  should  be 
spaced  not  less  than  twelve  inches  from  the  gauge-planes  of 
rails.  The  inner  guard-rail  should  l)e  6"  X  8",  l.iid  flat  and 
da|ip«'d  one  inch  on  the  ties.  The  outer  faces  of  tlic  inner 
guard  timbers  should  be  placed  five  or  six  inches  inside  the 
gauge-planes  of  rails.  Both  inner  and  outer  guard-rails 
.should  be  bolted  to  allernat!!  ties  by  three-quarler-inch  bolts, 
which  tuust  pass  through  the  stringers  also.  The  heads  of 
Uiese  bolts  should  be  countersunk  into  the  tops  of  the  guard- 
rails by  means  of  cup-shaped  washers. 

FRAMED   TUESTLKS. 

For  trestles  of  greater  height  than  thirty  feet,  and  for  less 
heights  under  certain  coiiditions,  it  will  be  necessary  to  use 
framed  bents.  The  foundations  for  these  maybe  i>rovided  by 
ilriving  piles  and  cutting  them  off  above  the  ground,  by  using 
timber  sills,  or  by  building  .small  masonry  piers. 

In  all  such  trestles  it  will  be  necessary  to  brace  the  struc- 
ture thoroughly,  both  transversely  and  longitudinally. 

All  framing  of  bents  should  be  done  in  such  a  manner  as  to 
tie  all  parts  firmly  together. 

For  very  high  trestles  it  will  l)e  economical  to  increase  the 
lengths  of  alternate  panels  to  twenty-five  or  cvpu  thirty  feet, 
And  truss  the  stringers, 


276 


DE    POXTIBra. 


The  lougitiidiniil  brnciiig  sliould  consist  of  diiiguiinlH  of  lim- 
ber of  suitable  (iiincnsiuns,  ill  alternule  puiiels,  witii  liuri/oii- 
tnl  struts  iimdc  continuous  iit  bracing  piincd  points  liirougliout 
all  panels. 

In  addition  to  the  transverse  and  longitudinal  bracing  pre- 
viously described,  all  trestles  on  sharp  curves  should  be  pro- 
vided with  a  lateral  system  composed  of  timber  diugoiials 
spiked  to  caps  and  to  bottoms  of  stringers. 

What  has  been  said  in  regard  to  lioorlng  for  pile- trestles 
applies  also  to  framed  trestles. 

The  compression-members,  when  impact  is  included  in  th : 
stresses,  are  to  be  proportioned  by  tlie  formula; 

j)  =  1700  -  0.4r  J 

for  long-leaf  yellow-pine  and  hard  woods,  and 


p  ~  1000  -  0.2 


for  white  pine,  short-leaf  yellow  pine,  and  soft  woods,  in 
which  forniulte  ^  and  />  are  respectively,  in  the  same  unit,  tlic 
unsupported  length  and  the  leiist  transverse  dimension  of  the 
strut. 


/ 


SPECIFICATIONS  FOR  TIMBER  TRESTLES. 

CLKAKINU   AWAY   UUHBIBU   AND   PUI<:l'AKI^(^   OIIOUNO   FOB 
STARTING   WORK. 

Before  beginning  work  on  any  trestle,  all  rubbish  'ogs. 
trees,  and  brush  must  be  cleared  awaj',  and  all  combustible 
material  must  be  burned  or  removed  for  the  entire  width  of 
the  right-of-way. 

DIMENSIONS 

All  posts,  braces,  stringers,  ties,  guard-rails,  sills,  and  all 
timber  generally,  shall  be  of  the  exact  dimensions  given  and 
Ji^ured  in  the  diawings.     No  variilions  fro:a  these  wjll  bp 


TIMBER  TRESTLES. 


2:7 


iiilowed.  except  upou  tUe  written  consent  of  the  Engiueer  or 
his  duly  authorized  lepreaeutative. 


/ 


DRAWINGS. 

The  drawings  will  be  made  to  the  scales  indicated,  but  in  all 
cases  the  tigures  are  to  lie  followed  in  preference  lo  the  scale, 
where  there  is  any  discrepancy  between  llie  two.  The  draw- 
ings are  lo  l)e  followed  exactly,  excepting  in  cases  of  errors  or 
omissions,  which  must  be  referred  to  the  Engineer  for  correc- 
tion or  for  additional  information. 

TIMBER. 

All  timber  shall  be  of  good  quality,  and  of  such  kinds  as 
the  Engineer  may  direct.  It  must  be  free  from  wind-shakes, 
wanes,  black,  loose,  or  unsound  ktiots,  sap,  worm-holes,  and 
all  descriptions  of  decay,  or  any  other  defect  wiiich  would 
impair  its  strenglli  or  durability.  It  must  l)c  sawed  true  and 
out  of  wind,  and  to  exact  dimensions.  Under  no  circum- 
stances will  any  timt)er  cut  from  dead  logs  be  allowed  to  be 
placed  in  any  portion  of  the  structure;  but  all  timber  must  be 
cut  from  living  trees. 

PILEB. 

The  piles  are  to  be  cut  from  good,  live  trees  of  such  varieties 
of  timber  as  may  be  selected  by  the  Engiueer.  They  must  be 
straight,  sound,  and  perfectly  free  from  wind-shake.i,  wanes, 
large,  loose,  black,  or  decayed  knots,  cracks,  worm -holes,  and 
all  descriptions  of  decay;  and  they  must  be  stripped  of  all  bark. 

If  square  piles  are  to  be  used,  they  must  be  hewed  sijuare 
and  not  sawed,  but  must  be  as  free  as  practicable  from  axe- 
marks.  Square  piles  must  be  at  least  twelve  (12)  inclies  across 
the  face,  and  must  show  not  more  than  two  (2)  inches  of  sap 
across  the  corners. 

The  sizes  of  round  piles  will  depend  upon  their  length,  but 
in  no  case  shall  they  be  less  than  nine  (9)  inches  in  diameter 
at  tips.  They  shall  be  so  nearly  straiglit  that  a  right  line, 
taken  in  any  ra<Iial  direction  and  running  parallel  to  a  riglit 
line  joining  the  centres  of  ends  of  pile,  shall  show  that  the  pile 


2rfi 


Dfi   I'ONtlhfS. 


1b  lit  no  point  over  oiu;  third  of  its  (liaiiiotcr  nt  siicli  point  out 
of  t\  struiglit  linu.  All  piles  iniiHt  hIiow  an  even  iind  grailiiul 
taper  from  end  to  end  ,  iind  the  tip  ends  lire  to  be  pointed  in 
im  approved  an*l  woikiniinlilie  nuinner.  Wherever  the  piles 
Hfu  liable  to  encounter  ln<rM,  bowlders,  or  any  olhir  material 
>vbieh  is  liable  lo  Hplit  or  injure  tliem,  liie  end»  are  to  be  pro- 
tected by  cast  or  wrought  iron  shoos. 

Whenever  iu  driving  it  becomes  apparent  that  the  hammer 
is  splitting  or  injuring  the  head  of  a  pile  lo  any  inaleriai  ex- 
tent, the  top  Is  lo  bo  banded  by  u  heavy  wrouglit-irou  ring 
while  the  pile  is  being  driven. 

All  piles  must  be  cut  otT  at  tops  to  an  evact  line  so  that  the 
caps  will  bear  evenly  on  all  the  piles  of  the  group. 

All  piles  injured  in  driving,  or  that  are  driven  out  of  place, 
shall  either  be  cut  oil  or  withdrawn,  as  the  Engineer  may  elect, 
and  others  shall  be  driven  in  their  stead. 

W^henever  the  heads  of  the  piles  are  of  greater  diameter  than 
the  width  of  the  caps,  they  are  to  be  adzed  olT  at  tlie  tops  at 
an  angle  of  about  forty-live  (45;  degrees,  so  as  to  be  Hush  with 
tiie  sides  of  the  caps. 

All  piles  must  bj  accurately  spaced  according  to  plans,  and 
those  beneath  the  track-stringers  must  be  driven  vertieaUy. 

All  battered  piles  must  he  diiven  to  the  angle  shown  on  ihe 
drawings.  Where  piles  of  dilt'erent  diameters  are  used  in  the 
same  bent,  the  large  piles  must  be  ad/ed  otT  wliere  tiie  diago- 
nal braces  cross  them,  sotliul  the  diagonals  will  not  be  bent  out 
of  Hue. 

FKAMING. 

All  framing  must  be  done  to  a  close  fit,  aiul  in  a  thorough 
and  workmanlike  manner.  No  bloeking  or  shimming  of  any 
kind  will  be  allowed  in  mal'iug  joints,  nor  will  any  open  joints 
be  accepted  anywhere  on  the  work. 

All  joints,  ends  of  posLs,  ends  of  piles,  etc..  and  all  surfaces 
of  timber  which  are  to  be  phiced  in  direct  contact  with  otUtiV 
timber  or  with  masonry,  must  be  thoroughly  painted  with  hot, 
creosote  oil,  and  then  covered  witli  a  good  coat  of  hot  asphal- 


TIMRKIl   TllKSTI-KS. 


2:9 


tiitn,  or  Hiiclt  olhur  iimteiiiil  ur  iiiiilfrial.s  as  iiiiiy  be  .selectfd  Uy 
the  Kngiiieer. 

All  lu»leH  of  Hiiy  kind,  wiiieli  me  bored  in  any  of  ihetlniberH, 
lire  to  be  tboroiiglily  Naiumted  with  hot  n-sitliuiliim,  and  all 
bolts  and  faiM  s  of  waHhei'H,  wliieh  are  to  be  placed  in  direct 
contact  V  I  the  timber,  are  to  be  warmed  and  dipiKid  in  a  vnt 
of  the  stuuv.  <unterial. 

The  hi  'i-'8  for  nil  bolts  of  three  quarters  fj)  of  an  inch  or 
more  in  diiiii;"ter  are  to  be  bored  one  eighth  (J^)  inch  less  in 
diameter  than  thai  of  the  bolls  \vlii<'h  are  to  be  used  in  them. 
For  smaller  bolts,  the  holes  are  to  be  bored  ouesixteeuth  (f\) 
inch  less  than  the  diameter  of  tiie  bolts. 

All  caps  are  to  be  thoroughly  drift-bolted  to  tops  of  i>lhs, 
All  bra(Mng  timbers  are  to  be  bolted  to  piles,  caps,  or  other 
timbers  wherever  they  cross  ilr.-m  The  ends  of  all  stringers 
shall  be  tirmly  attached  to  caps  by  means  of  drift-bolts,  timber 
cleats,  or  some  other  melliod  which,  in  the  opinion  of  the  En- 
gineer, is  ci-iuaily  gooil. 

For  slru(!turey  on  curves,  th ;  superelevation  of  outer  rails 
is  to  be  provided  for  by  bevelling  tlii'  ties,  not  to  exceed  three 
(8)  inches  in  tive  feet,  or  by  dapping  inner  stringers  on  caps 
not  to  exceed  two  (2)  inches,  or,  where  the  recpiired  .superele- 
vation is  too  great  to  be  provided  for  by  either  of  tlu'  two 
methods  named  or  by  a  combined  use  of  tliem,  by  cutting  oflf 
the  tops  of  the  piles  on  an  inclined  plane.  The  last  methou 
is  not  to  be  resorted  to  unless  it  be  absolutely  necessary,  and 
then  extreme  care  must  be  taken  to  cut  the  piles  off  so  that 
their  tops  will  lie  in  a  true  plane.  In  no  instance  is  tliis  to  be 
done  when  framed  bentsare  used,  as  the  inclination  can  then  be 
given  in  cutting  the  tops  of  the  bent  posts  to  receive  the  caps. 


METAL-WOIIK 

All  bolts,  nuts,  and  dowel- pins  shall  be  made  of  soft  steel  or 
wrought  iron  of  the  same  quality  as  that  specified  for  adjust- 
able members  of  bridges  iu  Chapter  XVIII.  Preference  will 
be  given  to  screw-bolts  of  soft  steel  with  coldpressed  threads. 

All  bolts  must  be  practically  perfect  in  every  rcspec  t,  and, 


280 


DE   PONTJBUS. 


wherever  necessary,  they  must  be  provided  with  uuts  and 
threads  of  the  standard  size  required  for  tlieir  diameter.  The 
thicliuess  of  a  nut  shall  not  be  less  thau  the  diameter  of  the 
rod  for  which  it  is  intended,  and  the  side  of  a  square  nut  must 
not  be  less  lh:in  twice  the  diameter  of  the  bolt.  The  heads  of 
all  bolts  shall  be  of  the  same  size  as  the  nuta  required  for  tlie 
screw  ends.  All  screw-bolts,  drift-bolts,  and  dowel-pins  shiill 
be  made  truly  straight  before  being  driven,  and  all  nuts  must 
be  screwed  up  light  against  the  washers.  All  nuts  and  heads 
of  bolts  must  have  heavy  O.G.  washers  between  them  and  the 
timber.  All  washers  are  to  be  made  of  cast  iron  of  good  qual- 
ity, and  must  be  sufficiently  large  and  thick  to  provid.  properly 
for  distributing  the  pressure  due  to  the  greatest  allowable  ten- 
sion in  the  bolts  over  the  area  of  the  washers.  They  must  bi; 
finished  in  a  neat  .'uul  workmanlike  munner,  and  must  be  free 
from  air-holes,  cracks,  cinders,  and  other  defects.  All  spacers 
are  to  be  made  of  cast  iron,  unless  otherwise  speciticd,  and 
must  be  of  the  same  quality  and  finish  as  specified  for  the 
washers. 


ERECTION,  DEFECTIVE  WOKK,  DIRECTIONS  TO  CONTRACTOR, 
CLOSING  THOROUGHFARES,  RESPONSIBILITY  FOR  ACCIDENTS, 
DA.VIAOER,  ALTERATION  OF  PLANS.  STRICTNESS  OF  INSPEC- 
TION, SPIRIT  OF  THE  SPECIFICATIONS,  ENOINKER,  AND  TEN- 
DERS. 

See  Chapter  XVIII. 


CHAPTER  XXI. 


INSPECTION  OF  MATERIALS  AND   WORKMANSHIP. 


Unless  all  tlie  iimterials  used  in  ti  stnictnic  and  all  work, 
matisbi])  during  tlie  Viirioiis  slages  uf  manufacture  ul  liie  shops 
and  of  constrUcliou  '".i  th«  field  be  subjected  to  conipetenl  and 
lionest  inspection,  much  of  the  benefit  obtained  by  scientific 
design  and  thorough  specifications  will  be  lost. 

For  many  years  most  of  the  inspection  of  structural  metal- 
worli  was  a  sad  f.'irce  ;  and,  in  consequence,  the  general  public 
placed  but  little  confidence  in  inspecti(m,  witli  t.ie  result  that 
a  large  portion  of  the  bridge-work  of  the  country  was  left 
entirely  to  the  tender  mercies  of  the  manufacturers,  who  nat- 
urally worked  for  their  own  interest  and  not  for  that  of  the 
purchasers.  Latterly,  however,  owing  to  the  efforts  of  a  few 
flnst-class  inspecting  bureaus,  the  status  of  inspection  has  been 
somewhat  improved,  although  it  is  far  from  bi  ing  to-day  what 
it  ought  to  be.  In  making  this  Itist  statement  the  author 
speak,  idvisedly,  in  that  he  has  suffered  considerably,  even  of 
late  years,  from  bad  inspection  in  such  matters  as  the  insertion 
of  a  rust-joint  in  a  turntable  between  the  bottom  of  drum  and 
top  of  upper-track  segments,  where  no  such  filling  was  allowed 
in  cither  plans  or  specifications;  badly  matching  holes  in  field 
conneciions;  pinholes  too  small  for  pins;  important  members 
omitted  in  shipping;  eye-bars  made  longer  than  called  for  by 
the  drawings  ;  great  recesses  In  webs  xnd  fillers  at  ends  of 
girders;  and  shop-paint  applied  over  half  an  inch  of  frozen 
mud.  Such  things,  to  say  the  least,  are  extremely  annoying, 
and  often  cause  great  expense  during  erection. 

Primarily,  the  blamo  for  such  errors  must  fall  on  the  in- 

261 


Jjs2 


DK   PONTIBTS. 


spcctors,  for  such  eirregious  bluuders  slioiiM  never  esc;a|)C' 
lluir  observttliou.  But  tliey  are  by  no  menus  entirely  to  blame 
for  the  fact  that  tlie  inspection  of  sU'uclunil  steel  in  gcMenil  is 
not  wliat  it  ought  to  be  ;  because  biidi  of  them  are  the  rtiilroad 
managers  and  promoters  of  birge  nnlerprises,  who  do  not  rec- 
ognize ihe  necessity  for  tirst-class  inspection,  and  wlio  are  often 
not  willing  to  pay  one  half  of  what  such  inspection  is  worth. 
Here  again,  though,  the  inspectors  are  to  blame,  for  the  rtascm 
that  in  the  keenness  of  their  competition  for  work  they  have 
cut  prices  to  such  an  extent  as  to  make  it  impossible  to  do 
proper  inspection  without  losing  money.  When  pinned  down 
to  facts  they  have  to  confess  this.  Tiie  coolnes.--  of  some  of  the 
"small  fry"  inspectors  is  often  amusing  The  author  was 
once  baided  over  the  coals  by  one  of  this  class  who  had  put  in 
a  low  bid  for  some  inspection,  and  whose  tender  had  been  re- 
jected because  of  the  low  figure,  the  work  having  been 
awarded  to  one  of  the  regular  inspecting  bureaus  at  about  fifty 
per  cent  more  than  the  unsuccessful  bidder  asked.  After 
expressing  his  mind  pretty  freely,  be  fired  this  parting  shot : 
"  Well,  I  never  intended  to  do  thorough  inspection  for  you, 
anyhow." 

The  inspection  business  has  been  utterly  demoralized  in 
times  past  by  just  such  action  as  that  contemplated  by  this 
inspector  ;  for  it  was  the  general  custon\,  and  is  yet  to  a  cer- 
tain extent  with  some  inspectors,  to  take  contracts  for  inspec- 
tion at  whatever  figures  the  purciiasers  are  willing  to  pay, 
then  handle  the  work  so  as  not  to  lose  money  on  the  contract, 
regardless,  of  course,  of  the  interests  of  their  em|)loyer8. 

Strange  tales  concerning  inspection  come  to  the  ears  of 
engineers— such,  for  instance,  as  passing  car-load  after  car-load 
of  metal-work  that  was  not  seen  by  .the  inspector  until  after 
loading  for  shipment ;  but  such  tales  need  verification,  which, 
of  course,  it  is  nobody's  business  to  give  them.  Tiiere  is  no 
doubt,  though,  of  some  of  them  being  authentic.  In  one  case 
)D  the  author's  experience  the  inspector  left  his  work  for  ten 
days  in  charge,  of  one  of  the  bndge-coinpant/'s  ahipping-clerka, 
without  notifying  either  the  author  or  his  direct  employers, 
the  inspection  bureau,  of  his  contemplated  absence.     8uch 


iNSl'ECTloy    OF    MATERfALS    ANM)    WOllKMA  XSHIP.    283 


nctions  us  this  inukc  one  entertuiu  doubts  soinetiincs  as  to 
whether  inspection  reiilly  pays. 

It  is  possible  that  the  general  deniorulizatioii  of  nietiil  iu- 
speclion  by  iiisufflcient  prices  aud  Iseeu  conipelilion  has  low- 
ered the  quality  tliereof  to  such  an  extent  that  even  tlie  highest 
possible  prices  would  not.  make  it,  for  some  time  to  come, 
what  it  ought  to  be  ;  because  not  only  are  the  assistant  in- 
spectors lucking  in  i)roper  tniiniiig  and  thoroughness,  but  the 
manufuclurers  have  become  accusloined  to  u  ceriaiii  class  of 
inspection,  and  would  deem  it  a  hardship  to  be  subjected  to 
much  more  rigid  requirements.  Eventually,  however,  the  re 
suiting  improvement  iu  mauufivcture  of  metal-work  would  be 
an  advantage  to  the  mauufrcturers  as  well  as  to  the  pur- 
chasers. 

A  decided  improvement  iu  inspection  can  be  brought  about 
only  by  concerted  action  on  the  part  of  the  principal  inspecting 
bureaus  and  inspectors  of  the  country,  backed,  of  course,  by 
tlic  aid  of  all  engineers  who  are  directly  interested  in  the  de- 
signing and  building  of  structural  metal  work.  If  these  in- 
specting bureaus  au<l  inspectors  of  established  reputation  were 
to  form  an  association  for  the  i)urpose  of  delermiidng  wlwit 
inspection  should  consist  of,  and  what  mininuuu  rates  sliould 
be  cliarged  therefor  by  all  members  of  the  association,  and  if 
admission  to  tlie  association  were  based  upon  both  experience 
ami  good  faith,  it  would  be  piacticable  to  make  very  quickly 
the  improvements  requisite  for  bringing  insi)ection  up  to  an 
almost  ideal  standard  of  excellence  For  a  while  a  good  dial 
of  work  would  go  to  the  inspectors  outside  of  the  association  ; 
but  ere  long  the  general  public  would  become  educated  to  the 
fact  that  good  inspection  of  metal-woik  is  a  necessity,  and 
that  it  cuu  only  be  obtained  by  paying  living  prices  to  those 
who  do  the  work. 

Engineers,  in  order  to  aid  in  the  good  work  of  the  associa- 
tion. 3hould  refuse  to  include  the  price  of  inspection  in  tlieir 
fees  for  engineering  work,  and  should  make  it  a  rule  to  em- 
ploy for  doing  their  iiisi)ection  only  members  of  the  associa- 
tion. 

Certain  engineers  of  high  standing  have  spoken  slighiingly 


284 


1>R   PONTIBCS. 


of  Ibis  propusiliou  to  form  nn  nssociation  of  inspectors,  term- 
ing it  a  "  trust."  Strictly  speaking,  it  certainly  would  par- 
take of  the  nature  of  n  trust,  but  it  would  bo  a  good  and 
worthy  one,  whose  main  ohj  ct  would  be  to  effect  a  much- 
needed  reform.  On  the  s:une  basis  the  American  Institute  of 
Architects  is  a  trust,  for  the  reason  that  it  establishes  a  miui- 
nnnii  fee  of  five  per  cent  for  the  making  of  plans  and  specifi- 
cations and  for  the  services  of  an  inspector  on  all  building 
work;  and  surely  such  an  organization  should  not  be  con- 
demned on  this  account.  On  the  contrary,  the  architects  have 
set  the  engineers  a  good  e.xuniplo  in  forming  this  association  ; 
and,  until  engineers  follow  their  lead  in  this  particular  and  es- 
tablish minimum  fees  for  professional  work,  the  engineering 
profession  will  fail  to  attain  its  higliest  degree  of  ctticiency, 
and  will  therefore  not  be  properly  recognized  as  a  profession 
by  the  general  pubMc. 

Returning  to  the  (piestion  of  the  inspection  of  stnxctural 
steel,  the  author  herewith  presents,  as  his  idea  of  what  good 
inspecticm  sliould  consist,  his  standard  instructions  to  the  iu- 
spccling  bureau  which  he  employs  and  to  its  inspectors  at 
mills  and  shops. 

First.  Study  carefully  the  engineer's  drawings  as  soon  as 
they  are  finished,  and  make  out  a  list  of  special  points  and 
features  that  will  require  e.xtra  ciive  in  the  shops  to  secure 
good  workmanship  and  proper  fitting,  then  niake  out  a  type- 
written report  of  these  and  submit  it  without  delay  to  th", 
Engineer. 

Second.  Study  carefully,  as  soon  as  they  are  finished  and 
approved,  all  shop  drawings,  so  as  to  beco.ine  thoroughly 
familiar  with  the  entire  construction. 

Third.  Make  sure  that  metal  of  uinform  character  and  of 
the  strength,  elasticity,  and  ductility  specified  is  furnished  by 
the  rolling-mills,  folio  vin^-  the  metal  from  one  process  to  an- 
other from  start  to  finish,  and  making  sure  that  the  test-pieces 
broken  represent  coirectly  the  metal  they  are  supposed  to 
represent. 

Fourth.  Check  tiie  chemical  analyses  of  the  metal  occasion- 
ally, so  as  to  gee  that  they  are  properly  made,  taking  care  th>it 


INSPECTIOiN'    OF    MATRRFALS    AND  WORKMAXSKIP.    285 


the  Contractor  is  Inforinetl  as  to  what  piece  the  samples  arc 
takeu  from,  so  that  lie  can  make  a  check  lest,  if  he  so  desire. 

Fifth,  See  lliut  all  the  various  tests  iiulicjilid  in  the  specifi- 
calioDS  are  made  faithfully,  the  iiunl)er  of  same  depending- 
upon  the  relative  uniformity  of  tiie  moljil  furnished. 

Sixth.  Make  sure  that  all  the  puncliing  is  done  with  sucii 
care  that  the  assembled  parts  will  come  together  so  as  to  mnku 
the  rivet-holes  match  so  accurately  that  when  the  reaming  is 
tinisheil  there  shall  be  no  imgidiir  holes. 

Seventh.  Make  sure  that  all  pieces  are  cut  to  exact  length 
and  proper  bevel,  that  all  web-stiffening  angles  bear  perfectly 
at  top  and  bottom  against  tlange  angles,  and  that  there  are  no 
loose  rivets. 

Eighth.  Wherever  rivels  with  flattened  heads  or  counter- 
sunk rivets  are  calle<l  for,  make  sure  that  they  are  properly 
chipped  or  otherwise  brought  to  correct  dimensions;  also  see 
that  the  ends  of  all  members  are  limited  to  the  lengths  beyond 
the  last  rivet  or  pin  hole  shown  on  the  drawings.  Give  par- 
ticular attention  to  the  eiuls  of  all  posts  and  chord-members  to 
see  that  the  "over-all  "  and  the  clear  diniensions  between  jaws 
correspcmd  faithfully  to  those  indicated  on  the  drawings. 

Ninth.  Take  some  elTective  means  of  ensuring  that  the  en- 
tire work  shall  go  together  properly  and  without  difficulty 
during  erection,  and  .so  that  when  completed  it  shall  conform 
in  every  particular  with  the  Engineer's  design,  even  if,  to  ac 
complish  the  same,  it  be  necessary  in  special  cases  to  assemble 
the  entire  work  at  the  shops. 

Tenth,  Watch  careftdly  the  punching  and  handling  of  tin; 
metal  in  the  shops,  so  as  to  see  that  no  cracks  develop  therein, 
and  that  the  metal  withstaiuis  properly  the  niaiupidation, 
showing  as  perfect  homogeneity  as  is  found  in  the  best  struc- 
tural steel. 

Eleventh.  Condemn,  as  soon  ?i3  it  is  tliscovcred,  any  material 
untit  in  the  slightest  degree  for  >ise  in  the  structure,  no  matter 
how  many  limes  it  may  have  already  been  in.spected  and 
passed. 

Twelfth.  See  that  all  metal-work  is  proi^erly  cleaned  by  the 
most  approved  methods  and  apparatus  bjjfore  the  tirst  coal  0/ 


2H6 


DE    PONTIHUS 


paiiil  is  applied,  and  tliiit  tlio  latter  is  allowed  to  dry  thor- 
oughly before  the  metul-work  is  loaded  ou  the  cars  for  ship- 
ment. 

Thirteenth.  See  that  all  shop  painting  is  thoroughly  done, 
and  that  proper  paint,  mixed  so  us  to  comply  with  the  specirt- 
cations,  is  invariably  used  ;  and  make  an  occasional  chemical 
analysis  of  the  paint,  taking  care  thai  the  Contractor  is  noticed 
of  the  contemplated  test  aftiir  tiie  samples  ary  taken,  in  order 
that  he  may  make  a  check  analysis,  if  lie  so  desire.  Take 
special  care  to  prevent  any  pieces  of  metal  from  being  riveted 
together,  unless  the  contiguous  faces  be  first  thoroughly 
painted. 

Fourteenth.  Insist  upon  the  discharge  of  any  employee  of  the 
Manufacturing  Company  who  wilfully  violates  or  continues 
to  violate  the  specifications  and  the  instructions  given  by  the 
Engineer  or  his  inspectors. 

Fifteenth.  While  endeavoring  in  every  possible  way  to  ob- 
tain good  work,  avoid  as  nuich  as  possible  doing  anytiiing  to 
annoy  or  harass  the  Contractor;  but,  on  the  contrary,  take 
special  pains  to  aid  him  in  every  legitimate  manner  to  finish 
his  work  quicUly  an. I  inexpensively. 

Sixteenth.  Formulate  and  prepare  for  each  large  piece  of 
work  the  best  practicable  method  of  recording  progress  and 
reporting  thereon,  and  divide  up  tiie  total  work  into  groups  or 
sections,  so  tiiut  the  notes  may  be  easy  for  reference.  Tliis 
should  be  done  by  the  inspecting  bureau,  and  should  not  be 
left  to  the  shop  inspector. 

Seventeenth.  Send  into  the  office  of  the  Engineer  regular 
weekly  reports  concerning  the  progress  of  the  work,  any 
special  reports  that  from  time  to  time  appear  to  be  recpiired, 
the  tabulated  results  .of  all  tests  of  materials,  and  copies  of  all 
.shipping  bills. 

Eighteenth.  Make  sure  that  all  shipping  weights  are  correct 
by  seeing  the  metal  weighed,  and  keep  account  of  the  weight 
©fall  metal  sent  out  on  the  work,  as  the  Contractor  will  be  paid 
by  the  pound.  It  will  be  necessary  for  the  inspecting  bureau 
to  check  all  of  these  weights  against  the  shop  drawings  to 
show  how  they  agree  or  disagree.     A  detailed  statement  o.f 


TNSI'KCTIOX    OF    MATKKIALS    AND  WOWKSfANSlIIP.    287 


both  sots  of  weights  must  be  sent  to  the  Engineer  upon  the 
coinpic'tiou  of  tlio  contract,  or,  at  his  request,  upon  the  com- 
pletion of  an}'  (Jelinito  portion  tliereof. 

Nineteenth.  Tiie  inspecting  bureau  shall,  under  no  ciroum- 
stances  whatsoever,  intrust  respousil)le  work  of  any  kind  to 
iiisufticiently  trained  assistants.  Wiien  new  inspectors  are  to 
be  broken  in,  they  must  receive  their  trainii  g  in  such  a  way 
as  not  to  jeopardize  in  tiie  slightest  degree  the  quality  of  the 
material  or  workmanship. 

Twentieth.  Finally,  and  in  short,  do  all  you  can  to  make  the 
structure  in  every  sense  of  the  word  a  credit  to  uU  concerned 
in  its  designing  and  construction. 

The  author  has  had  m-ide  for  him  lately  by  Mr.  11.  T.  Lewis, 
one  of  his  inspectors,  a  rather  interesting  series  of  tests  to  de- 
termine the  average  accuracy  of  punched  rivet-holes.  These 
tests  were  made  after  the  metal  w.is assembled  for  reaming  by 
inserting  rods  of  various  diameters  in  the  assembled  holes. 
From  the  results  of  these  tests  the  author  has  prepared  the 
following  clause  for  the  speciticalions  given  in  Chapter  XVIII. 

"All  pupphed  work  shall  he  so  accurately  done  that,  after 
the  various  component  pieces  are  assembled  and  l»jfore  the 
reaming  is  commenced,  forty  (40)  per  cent  of  the  holes  can  be 
entered  easily  by  a  rod  of  one  si.xteenth  (^jf)  of  an  inch  less 
diameter  than  that  of  tiie  punched  holes;  eighty  (80,  per  cent 
by  a  rod  of  a  diameter  one  eighth  (i)  of  an  Inch  less  than  same; 
and  one  hundred  (100)  pir  cent  by  a  rod  of  a  diameter  one 
(pnirler  {\)  of  an  inch  less  than  same.  Any  shop-work  not 
coming  up  to  this  requirement  will  be  subject  to  rejection  by 
the  inspector." 

It  will  be  noticed  that  this  specilicaiion  does  not  reject 
absolutely  all  work  that  does  not  come  up  to  its  exact  require- 
ments, the  inspector  being  allowed  some  latitude  in  distin- 
guishing between  simple  and  complicated  sliop-work,  imi)or- 
tant  and  unimportant  connections,  and  the  assembling  of  few 
and  of  luinu'roiis  component  pieces. 

If  the  Association  of  Inspectors  herein  suggested  were  estab- 
lished, it  could  do  good  work  for  the  engineering  professiou 
hy  laying  out  a. series  of  tests  of  full-sized  jDciubersautJ  details 


288 


DE   PONTIBUS. 


of  bridges  and  other  stnictunil  metal-work,  to  be  made  from 
time  lo  time  as  a  portion  of  tlie  inspection  for  large  contracts. 
Tliis  would  need  tlie  assistance  of  tlie  consulting  engineers, 
who,  In  preparing  their  specifications,  sliould  include,  as  a 
part  of  the  work  of  the  manufacturers,  the  making,  under  the 
supervision  of  the  inspectors,  of  certain  tests  of  full  size  parts, 
it  being  understood  at  tlie  outset  that  the  results  of  such  t«'8tF 
shall  be  of  direct  value  to  the  accomplishment  of  the  work 
covert  il  by  the  specifications  The  author  has  for  the  past  five 
years  been  endravoiing  in  this  way  to  obtain  some  much 
needed  information  concerning  tlie  strength  of  both  main 
members  and  details  of  bridges  and  elevated  railroads;  but  his 
attempts  to  have  the  tests  mtide  have  not  always  proved  suc- 
cessful 

As  foi-  the  p»*oper  price  lo  pay  for  firsl-class  inspection,  the 
author  would  slate  that  some  tliree  years  ago  he  s«d)mitted  to 
several  of  the  principal  inspecting  liureaus  a  draft  of  instruc- 
tions to  inspectors  <il  mills  and  shops,  similar  to  those  incor- 
])orated  in  this  chapter,  with  a  rtHpiest  that  they  tender  upon 
inspecting  for  him,  according  to  said  instructions,  u  large 
order  of  structural  steel  ;  and  that  the  bids  received  varied 
from  one  dollar  to  one  dollar  and  twenty-five  cents  per  ton  of 
twi)  thousand  pounds.  Subsequent  experience  has  proved  to 
the  author  that  such  inspection  as  he  then  called  for  is  worth 
fully  one  dollar  per  ton  for  large  orders  and  a  tritle  mqre  for 
smaller  ones;  although  il  is  very  seldom  that  such  a  price  is 
paid  in  this  country  for  inspection. 

In  respect  to  inspection  of  materials  and  workmanship  in 
the  field,  the  following  instnictions,  which  the  author  liaspre- 
l)ared  for  his  field  forces  of  engineers  and  inspectors,  will  be 
found  to  cover  the  subject  pretty  thoroughly. 

(a)   METAIi-WOUK. 

First.  Examine  with  the  greatest  care  .",11  of  the  metal-work 
as  fast  as  it  is  delivered,  so  as  to  make  sure  that  it  lias  not  been 
injured  during  transportation,  and  keep  an  eye  on  it  there- 
after to  see  that  it  is  not  injured  during  erection.  See  also 
that  there  are  no  missing  parts. 


INSFE(JTION    OF    MATEK1A1>S    AND  WUKKMAN.SHIl'.    ;280 


Secoud.  See  thai  the  metal-work  goes  together  properly  and 
expeditiously,  and  report  to  the  Engineer  all  necessity  for 
chipping  or  filing  on  account  of  bad  shop-work. 

Third.  Watch  carefully  the  riveting  to  see  that  no  burnt 
rivets  are  used,  that  all  field-rivets  are  driven  in  accordance 
with  the  specifications,  and  that  uo  loos*'  rivets  are  left  in  the 
work. 

Fourth.  See  that  all  vacant  spaces  in  the  nietal-woik  are 
completely  filled  with  paint-skins  or  other  water-proof  male- 
rial  before  the  painting  is  begun 

Fifth.  In  elevated-railroad  work  see  that  during  the  erec- 
tion of  the  metal- work  the  lengtlis  of  the  girders  are  sutliciently 
correct  to  prevent  all  po.ssihility  of  using  up  the  spaces  pro- 
vided for  expansion,  assuming  tlie  greatest  temperature  of  the 
metal  to  be  one  liundred  and  twenty-five  degrees.  See  also 
that  the  expansion  and  contraction  of  the  structure  cannot  in- 
jtire  the  stairways. 

Sixth.  In  drawbridges,  see  that  the  masonry  of  the  pivot- 
pier  is  levelled  off  with  the  greatest  accuracy,  and  that  the 
lower  track-segments  are  set  to  exact  position  and  level,  thus 
making  a  perfectly  conical  surface  for  the  r;)ller8.  See  also 
that  the  latter  are  adjusted  so  as  to  bear  evenly  at  top  and  bot- 
tom against  both  upper  and  lower  track-sei^mcnts. 

Seventh.  See  that  the  ends  of  draw-^nans  are  properly 
adjusted  by  means  of  the  shimmiiig-philes  on  the  rest  piers 
and  those  in  the  bottom  chords  near  the  pivot-pier.  Make 
sure  that  in  every  particular  the  draw  is  veversible  end  for 
end;  and  see  that  all  shafting  is  properly  aligneu  .othat  there 
will  be  uo  binding  in  any  of  the  bearings. 

Eighth.  See  that,  before  the  operating  machinery  is  tested, 
all  slidingor  rolling  surfaces  are  thoroughly  lubricated,  and 
that  the  turntable  is  cleared  of  all  obstructions,  such  as  nails, 
etc.,  on  tl"  lower  track-segments.  Then  make  a  ti.'st  of  the 
machinery  and  compute  therefrom  the  horse-power  required 
to  operate  the  draw. 

Ninth.  See  that  all  anchor-bolts  are  set  in  exact  position  and 
to  correct  level,  and  that  they  are  projjerly  grouted  in. 

Tenth.  In  placing  the  bearings  for  arches,  take  tlie  grc/i] 


290 


DE   PONTIBUS. 


est  cnre  that  the  ceiilres  are  set  to  exact  position  and  level, 
and  that  the  bearings  for  the  metal-work  on  the  masonry  are 
perfect. 

Eleventh.  Whenever  there  are  any  atlju.stal)le  rods  used  In 
a  structure,  see  llmt  they  are  properly  tightened  before  the 
work  is  left,  taking  care  thai  they  are  not  screwed  up  more 
tightly  than  is  really  necessary. 

(b)  kailb. 

First.  Examine  all  rails  us  soon  as  received,  so  as  to  see  that 
there  are  no  poor  ones  which  have  escaped  the  rail-inspector's 
eye,  or  which  have  been  loaded  for  shipment  after  being  re- 
jected. Inspect  also  all  other  track-metal,  such  as  angle-bars, 
bolts,  and  biiices,  so  as  to  see  that  they  are  of  the  correct  type 
and  are  delivered  in  good  shape. 

Second.  See  that  all  rails  are  l;iid  to  exact  line  and  levi-l, 
and  that  they  bear  properly  everywhere. 

Third.  In  draw-spans,  make  sure  that  the  track-rails  at  the 
ends  will  not  interfere  with  the  operation  of  the  draw. 


(C)   PAINTING. 

First.  See  that,  after  proper  cleansing  and  retouching  with 
paint,  the  metril-work  receives  its  first  field-coat  of  paint  as 
soon  as  practicable  after  erection,  and  that  the  next  coat  is 
applied  as  soon  as  practicable  after  the  first  field-coat  is 
thoroughly  dried,  but  in  no  case  before. 

Second.  Make  sure  that  all  paints  used  are  of  the  proper 
color,  qualit}',  and  consistency,  and  that  no  adulterants  or 
thinners  are  used  ;  also,  that  all  paint  is  properly  applied. 

Third.  Look  carefully  to  the  painting  of  all  close  spaces 
between  metal,  and  see  that  it  is  done  efTeclively  with  a  piece 
of  cloth,  according  to  the  specifications. 

Fourth.  See  that  all  jtortions  of  the  metal-work,  which  are 
to  rest  on  the  masoiny  or  which  are  to  bo  embedded  in  the 
concrete,  receive  their  two  field-coats  of  paint  in  due  time,  so 
as  to  dry  thoroughly  before  the  said  metal-work  is  erected. 


INSPKCTION    OF    MATEIUALS   AND  WOKKMANMim'.    291 


{O)   EXCAVATION. 

First.  Watch  carefully  all  excavation  so  as  to  make  sure 
that  it  is  (lone  in  strict  acconinncu  with  the  spcciflcntioiis  and 
witii  the  City  Ordiiiaucus,  if  there  be  any.  See  tliat,  in  doing 
the  exciivalioM  and  in  building  the  structure,  the  Contra(;to.r 
does  not  obstruct  public  tralHc. 

Second.  In  foundation-work  in  cities,  see  tliat  all  pipes  and 
sewers  are  nu)ved  properly  and  coupled  or  spliced  effectively 
after  being  uncoupled  or  cut, 

'{''liinl.  Whenever  there  is  any  doubt  about  the  proper  re- 
sistance of  any  foundation,  test  it  by  loading  it  by  means  of  a 
pn»porly  designed  and  built  apparatus.  Always  ram  thor- 
«)ughly  any  foundation  wliere  liie  resistance  to  loud  would  be 
increased  by  such  rauuning.  See  tliat  the  materiul  from  the 
sides  of  (he  pits  is  prevented  from  falling  in. 

Fourth.  See  that  all  surjilus  material  is  removed  expedi- 
tiously from  City  streets,  and  that,  whenever  any  piece  of  con- 
struction is  completed,  all  falsework,  rubbish,  etc.,  are  re- 
moved from  the  site  and  are  deposited  in  an  unobjectionable 
place. 


i(!h  are 
in  the 
me,  so 
Led. 


(e)  foundations. 

Firet.  See  that  the  bed-rock  is  always  properly  prepared  to 
receive  the  caisson  or  masonry,  as  the  case  may  be,  letting  the 
caisson  into  the  rock  so  as  to  provide  an  even  bearing  around 
the  cutting  edge,  and  levelling  or  stepping  off  or  filling  up 
with  concrete  to  receive  the  latter. 

Seconil.  In  elevated-railroad  work,  see  that  wherever 
colmnns  are  located  in  the  street  their  feet  are  properly  en- 
cased in  concrete,  and  that  cast-iron  fenders  are  correctly  set 
around  the  columns  and  filled  with  concrete  and  grouting,  (hen 
sealed  effectively  against  the  ingress  of  water.  See  also  that, 
after  the  columns  are  tip  ami  encased,  the  pavement  is  relaid 
in  a  substantial  manner,  to  the  satisfaction  of  the  City  au- 
thorities. 

Third.  When  large  steel  cylinders  are  used,  see  that  the^ 


292 


DE  poNTiiura. 


n re  kept  well  braced  with  limbers  on  tlio  inside  durinj,' sin K- 
ing,  HO  lis  to  avoid  all  possibility  of  collapse. 

Fourtli.  Soe  tliiit  proper  gindes  arc  provided  for  all  cais- 
sons and  cylinders,  so  that  Ihey  can  l)e  kept  in  exact  Iiorixontal 
position  during  the  entire  linking. 

Fifth.  Sec  that  the  tops  of  all  pit;rs  arc  properly  tliMslied  off 
to  receive  the  snperstructnrc,  taking  care  lliul  all  bearings  are 
made  perfectly  smooth  and  to  exact  level. 

(f)  caissons. 

First.  In  l)uilding  timber  caissons,  see  that  the  plans  are 
followed  exactly,  and  that  the  full  ({uantum  of  timber  bolls  is 
used  ;  also,  thai  short  timbers  are  not  put  in  where  long  ones 
are  called  for.     See  that  all  timbers  are  properly  framed. 

Second.  In  sinking  caissons,  see  that  tlicy  are  never  allowed 
to  deviate  matoriidly  from  correci,  position,  and  that  all  errors 
of  position  are  corrected  as  soon  as  oossible  after  they  are  dis- 
covered. 

Tliird.  In  (iliing  working-chambers  of  caissons,  see  that  the 
concrete  is  packed  tightly  against  the  roof,  and  that  no  voids 
whatsoever  are  left  therein. 


(a)   CEMENT   AND   CONCHiSTE. 

First.  Test  all  the  cement,  according  to  the  special  instruc- 
tions therefor,  so  long  before  it  is  needed  for  use  that  the 
Contractor  shall  not  be  delayed  by  such  testing. 

Second.  See  that  all  cement  is  housed  .so  as  to  be  protected 
effectively  from  the  weather,  and  that  no  dampened  or  other- 
wise injured  cement  is  allowed  to  be  used  on  the  work. 

Third.  Inspect  as  soon  as  delivered,  and  if  possible  before 
it  is  dumped  on  tlio  ground,  all  sand  and  broken  stone,  so  as 
to  make  sure  that  they  comply  in  every  particular  with  the 
specifications ;  and  insist  always  upon  all  of  these  materials 
that  are  rejected  being  removed  immediately  from  the  vicinity 
of  the  bridge  site. 

Fourth,  See  that  strong  and  proper  forms  for  concrete  are 
wspd  in  the  cpusiruclirn  of  all  j[)cdestals,  and  that  all  visjbJii 


INSPECTION'  OP   MATERIALS  AND  WOUKM ASSlIII'.    293 

portions  of  tUe  hitter  uic  tinislicd  oil  smooth,  tiiu  top  Hiiiface 
hi'iiig  hroiijjht  to  exact  elevation  and  made  perfectly  level. 

Fifth.  See  tliat  all  concrete  is  mixed  accordiiiij  to  the  speci- 
lications,  that  it  is  put  in  place  inunediately  after  mixing,  aud 
that  it  is  thoroughly  rammed. 

Sixth.  See  that  no  injury  is  done  to  the  concrete  in  remov- 
ing the  timber  forms,  or,  if  any  be  done,  that  it  be  properly 
repaired  ;  also,  that  the  timber  be  left  in  whenever  its  removal 
would  tend  to  injure  the  work. 

Seventh.  When  concrete  is  placed  under  water,  .sec  that 
either  a  tremie  or  proper  collapsing-bucket  be  used,  and  that 
the  water  be  not  permitted  to  injure  the  concrete.  See  al.so 
tliat  all  such  concrete  is  mixed  extra  rich. 

(ll)  IMLINQ   AND   TUKSTLEWOUK. 

First.  See  that  all  piles  conform  in  size,  quality,  and 
straightness  with  the  requirements  of  the  specifications,  even 
if  they  have  been  already  passed  by  the  timber  inspector  be- 
fore shipment,  and  reject  any  that  are  unfit  for  use. 

Second.  See  that  all  piles  ai'c  driven  straight  and  in  proper 
position,  and  that  the  tops  are  not  unduly  injured  in  driving 
having  the  said  tops  banded,  whenever  necessary  to  prevent 
splitting. 

Third.  See  that  all  piles  are  cut  off  at  the  exact  elevation 
required,  and  that  the  caps  are  properly  drift-bolted  thereto. 
On  curves  see  that  the  superelevation  is  obtained  properly, 
and  not  by  shimming  up  on  the  caps. 

Fourth.  See  that  all  sway-bracing  is  bolted  effectively  to 
the  piles  and  caps. 


(I)  TIMBER,   FLOORING,   AND  HAND-RAILS. 

First.  Inspect  all  timber  as  soon  as  delivered,  marking 
plaiidy  all  rejected  pieces ;  and  see  that  all  such  pieces  are 
removed  from  the  vicinity  without  delay,  in  order  to  prevent 
their  being  put  into  the  structure  without  the  knowledge  of 
the  resident  engineer.     It  is,  of  course,  permissible  to  use  the 


^94 


DE  PONTIBUS. 


good  portions  of  rejected  timbers  ;  but  iu  doing  so  great  rare 
should  be  exercised  to  prevent  tlie  workmen  from  putting  uiiy 
poor  materiiil  into  tiie  work.  The  fact  tluit  all  the  timber  re- 
ceived had  been  previously  uccupted  by  tlie  timber  inspector 
is  no  reason  for  using  unsatisfactory  material  ;  moreover, 
sometimes  it  happens  that  ti  nbers  which  the  inspector  iins 
never  even  seen  are  marked  with  his  stamp  and  siiipped. 

Second.  See  that  the  floor  system  is  properly  laid  and 
attached  to  the  metal-work,  that  each  rail  bt-ars  effectively 
upon  every  tie  which  it  crosses,  and  that  the  rails  are  laid 
stiaight,  evenly,  and  to  exact  grade. 

Third.  See  that  the  hand-railing  is  brought  to  proper  align- 
ment,  and  is  held  there  in  a  permanent  manner. 

Fourth.  See  that  all  joists  in  highway  bridges  are  properly 
dapped  on  floor-beams  so  as  to  bring  all  of  their  uppur  sur- 
faces to  exact  elevation  or  elevations  ;  also,  lliat  all  inter- 
mediate joists  lap  past  each  other  far  enough  to  reach  entirely 
across  the  top  flanges  of  the  floor-beams.  See  that  the  outer 
lines  of  joii^ts  abut  and  run  continuously,  and  that  they  are 
effectively  spliced  on  the  inside. 

Fifth.  See  that  all  joists  in  which  the  depth  exceeds  fo;ir 
times  the  thickness  are  bridged  at  distances  not  to  exceed 
eight  feet,  and  that  when  the  hand-railing  depends  for  its 
rigidity,  upon  that  of  llie  outer  joists  the  latter  are  well 
bridged  and  otherwise  stiffened  where  the  posts  are  attnched. 

Sixth.  See  that  alternate  bolts  attaching  guard-rtiils  to 
floor  pass  through  both  the  flooring  ami  the  outer  joists,  and 
that  all  holes  through  the  latter  are  bored  in  the  central  plane 
of  tlie  joist. 

(j)   MASONRY. 

First  Inspect  all  stone  as  soon  as  received,  so  as  to  see  that 
It  has  not  been  injured  in  transit,  and  that  it  is  satisfactory  in 
every  particular,  even  if  it  lias  already  been  passed  by  the 
stone  inspector. 

Second.  See  that  all  stones  are  thoroughly  cleaned  and  wet 
before  being  laid. 

Third.  See  that  all  mortar  is  mixed  in  the  proper  propor- 


IJJSPRCTIO^f   OF   MATKRIALS   AND  WORKMANSHIP.    S<)5 

lion,  and  that  it  is  used  ou  the  work  belore  any  set  has 
occurred. 

Fourtli.  See  that  all  joints  are  thoroughly  flushed  with 
niortiU',  and  that  no  voids  are  left  anywhere  in  tlie  masonry. 

Fifth.  See  tliat  all  coping-sloues  are  set  so  that  the  lop  of 
the  pier  will  lie  in  a  truly  horizontal  plane,  and  that  they  are 
kept  in  place  by  proper  clamps  and  dowels  as  per  plans. 

Si.xth.  See  tiiat  the  exposed  joints  are  all  cleansed  and 
jiointed  in  a  thorough  and  workmanlike  manner,  and  in 
accorduuce  with  tlie  specilications. 


(k)  geneual  instructions. 

First.  See  that  all  proper  precautions  against  accidents  to 
the  public  and  to  the  workmen  be  taken  during  erection,  and 
that  no  glaringly  careless  man  be  allowed  on  the  work. 

Second.  If  there  be  more  than  one  Contractor  on  the  work, 
see  that  no  friction  arises  between  contractors,  and  that  their 
combined  work  is  flnish6(l  in  good  shape. 

Third.  WiuMe  doing  everything  in  your  power  to  obtain 
gooil  work,  avoid  as  much  as  possible  worrying  or  hat assing 
the  Contractor,  and  use  every  legitlr'iate  endeavor  to  aid  him 
to  complete  his  work  expeditiously  and  inexpensively. 

Fourth.  Finally,  and  in  short,  study  the  specifications  care- 
fully, and  do  all  that  you  can  to  insure  the  structure's  being 
in  every  respect  a  credit  to  all  concerned  in  its  desigidng  and 
construction. 

In  respect  to  the  testing  of  cement  on  construction  work, 
the  following  iiiNtruclions,  which  the  author  has  pre[);ired  for 
his  resident  engineers,  will  give  the  reader  all  necessary  infor- 
mation, it  being  understood  that  no  brands  of  cement  are  ever 
used  except  those  which  either  the  author  or  his  a.s.sistaut3 
have  previously  tested  thoroughly  by  long-time  tests,  and 
which  have  proved  to  be  perfectly  satisfactory  : 

First.  In  testing  cenient  in  the  tield,  remember  that  it  is  not 
a  series  of  laboratory  tests  which  you  are  to  make,  but  that 
your  object  is  simply  to  see  that  you  are  receiving  and  using 
cement  of  an  average  quality  of  the  standard  brand  or  brands 


296 


DE   PONTIBUS. 


adopted,  and  that  it  comes  up  to  the  geueral  requirements  of 

the  specitications. 

Second.  Look  out  for  irregularities  in  the  quality  of  the 
cement,  so- as  lo  avoid  using  any  that  is  either  loo  old  or  too 
fresh,  or  wliich  has  been  iMJuietl  by  dampness. 

Third.  Test  tirst  for  fineness,  second  for  soundness,  third 
for  activity,  and  fourth  for  rise  iu  temperature,  rejecting  all 
cement  which  is  imtit  for  use  liecause  of  non-compliance  with 
the  specifications  in  these  particulars. 

Fourth.  It  will  seldom,  if  ever,  be  necessary  to  resort  to  the 
boiling  test,  which  is  essentially  a  laboratory  test  ;  although  it 
may  prove  useful  iu  an  emergency  to  determine  conclusively 
whether  certain  cement  is  fit  for  use  or  not. 

Fiflii.  Test  all  cements  for  the  ten.sile  strength  of  neat 
briquettes,  making  one  day  and  seven-day  tests.  Never  pass 
cement  on  shorter  time-tests  than  seven  days,  as  the  one-day 
test  is  by  no  means  conclusive. 

Sixth.  Make,  more  for  your  own  satisfaction  than  for  any 
other  reason,  a  few  sandbiiquette  tests  for  seven  and  twenty- 
eight  day.s,  so  as  to  know  the  value  of  the  mortar  which  you 
are  using.  It  would  not  do  to  rely  on  sand-briquette  tests  for 
the  acceptance  or  rejection  of  cement,  as  this  would  delay  the 
work  too  much. 

Seventh.  You  will  often  have  to  use  your  judgment  about 
passing  or  rejecting  cement  that  is  needed  for  immediate  use 
and  which  falls  in  some  compaiativel}'  unijuportant  point  to 
quite  fill  the  reciuirements  of  tlie  specitications.  Rather  than 
delay  the  Contractor  materially,  pass  such  cement,  provided 
that  in  your  opinion  its  use  will  iu  no  way  injure  the  quality 
of  the  work  ;  but,  on  the  oilier  hand,  if  the  rejection  of  the 
saitl  cement  will  not  delay  the  Contractor  seriously,  in.sist  on 
its  complying  with  the  specifications  in  every  particular.  Be 
careful  not  to  let  the  Contractor  run  iu  any  poor  cenienl  or 
force  it  upon  you  because  of  any  assumetl  or  real  necessity  for 
haste  in  completing  the  construction. 

In  respect  to  inspection  of  stone  for  masonry,  the  author 
offers,  as  his  idea  of  what  stone-inspection  should  be,  the  fol- 
lowing instructions  to  stone-inspectors,  it  being  understood 


INSPhCTION    OF    MATKUIALS   AND  WORKMANSHIP.    '^D"? 


that  they  apply  only  to  stone  from  quarries  that  have  been 
previously  investigated  and  found  satisfactory: 

First.  Ileject  all  stone  conlaining  iinj'^  dry  seams.  These 
seams  are  often  very  hurd  to  delect ;  but  u>ually  l)y  a  cnreful 
inspection  of  the  surface  of  the  stone  they  may  he  found. 
Sometimes  a  mere  line  is  all  the  evidence  of  the  existence  of 
such  seams,  while  in  olhe''  cases  they  show  nion;  jilainly. 

Second.  Reject  all  stone  containing  seams  called  "crow- 
foot," which  are  either  open,  or  which  are  liable  to  dissolve 
out  after  exposure  to  the  weather. 

Third.  See  tiiat  no  stone  is  quarried  at  a  time  when  it  is 
liable  to  freeze  before  the  quarry-sap  is  out  of  it.  Stone 
sliould  be  quanied  at  least  a  montli  before  it  is  allowed  to 
freeze. 

Fourth.  See  that  no  powder  or  other  explosive  is  used  in 
quarrying  the  stone,  excepting  to  remove  ledges  of  useless 
stone,  and  even  then  make  sure  that  no  stone  to  be  used  is 
injured  by  the  explosives. 

Fiftli.  If  the  stone  be  of  such  a  character  that  tlie  quarry, 
bed  cannot  be  told  at  a  glance,  the  Inspector  must  nuirk  eacli 
stone  in  such  a  manner  that  it  will  be  sure  to  be  laid  in  the 
wall  on  the  .said  quany-bed. 

Sixth.  Reject  all  stone  whicli  is  taken  from  any  portion  of 
the  quarry  that  is  affect  el  injuriously  at  any  lime  by  frost. 

Seventh.  See  that  all  stone  is  handled  carefully  after  being 
taken  from  the  quarry,  so  that  no  cracks  are  developed  or 
other  injury  done  thereto  by  rough  usage. 

Eighth.  See  that  all  stones  arecut  to  the  exact  dimensions 
called  for  by  the  plans,  and  that  they  comply  in  every  partic- 
ular with  the  specilications. 

In  respect  to  inspection  of  limber,  both  in  the  woods  and  at 
the  sawmills,  the  author's  instructions  to  his  timber-inspectors, 
as  follows,  will  be  found  useful : 

First.  Study  well  and  compare  with  the  mill  pet)ple  all 
order-bills,  looking  carefully  to  the  various  lengths,  widths, 
thicknesses,  bevels,  numbers  of  pieces,  etc.,  so  as  to  make  sure 
that  your  order-bills  check  properly  against  those  furnished 
to  the  mill  people  and  against  the  partial  order-bills  furnished 


298 


DE  PONTIBirS. 


by  the  latter  to  their  various  employees,  so  as  to  avoid  all  pos- 
sibility of  errors.  If  any  be  fomid,  correct  them  yourself,  if 
possible;  but,  if  imt,  refer  them  to  the  Eiigiueer  for  correc- 
tion. 

Second.  Each  timber-inspector  is  to  be  provided  with  a 
special  stamping  jpimmer  of  his  own,  that  lias  a  characleristie 
mark  which  will  identify  all  timber  passed  by  him.  He  is  to 
keep  this  hammer  at  uU  times  in  his  own  possession,  so  th&t  it 
can  be  put  to  no  illegitimate  use  by  interested  parties ;  and 
under  no  circumstances  is  he  to  lend  it  to  another  inspector. 

Third.  Each  timber-iMsi)ec'tor  must  study  carefully  the 
8|MJcifications  furnished  him,  and  must  be  governed  thereby; 
nevertheless,  there  will  be  occasions  when  he  must  trust  to 
his  own  judgment  as  to  what  timber  is  tit  and  what  is  until  for 
the  recpiired  purpose,  f<n  general  specifications  cannot  l)e 
made  broad  enough  to  cover  all  cases  that  may  arise  in  tilling 
a  timber  bill.  Where  a  number  of  inspectors  are  employed  on 
the  same  piece  of  work,  it  will  be  necessary  at  the  outset  for 
the  Chief  Inspector  to  interpret  the  specifications  and  supple- 
mentary instructions  for  all  of  the  assistant  inspectors,  so  that 
the  latter  .shall  not  dill'er  at  all  in  tlieir  requirements. 

Fourth.  In  inspecting  timber  be  careful  to  distinguish  piop- 
erly  between  the  various  varieties  that  are  tit  and  tlio.se  that 
are  until  for  use.  If  not  otherwise  stated  in  the  specitications, 
you  are  to  accept  and  reject  as  follows  . 

OAKS. 

Accept  white,  cow,  chiucapin,  post,  burr  or  overcup,  ami 
live  oaks.  Reject  red,  Spanish  or  water,  black,  black-jack, 
and  pin  or  yellow  butt  oaks. 

PINKS. 


AccejH  white,  Norway,  long-leaf  Southern  yellow,  short- 
leaf  yellow  (for  certain  purposes  only),  and  Cuban  pines;  also 
Oregon  fir.  Reject  Southern  red,  loblolly,  and  Rocky  Moun- 
tain yellow  pines. 


INSPECTIONT   OP    MATKRIAL8   AND  WOHKMANSHIP.    290 


CYPRESS. 


Accept  red,  black,  and  yellow  cypress.  Reject  white  cy- 
press. 

Fifth.  Secure  timber  of  as  uniform  a  character  as  possible, 
avoiding  any  that  shows  large  heart-checks  or  growtli-chcclis, 
aud  rejecting  any  wliich  has  sucli  defects  within  one  incli  of 
face  or  edge  of  timber.  Avoid  nil  coarse-growtli,  open- 
grained  timber,  if  otlier  timber  be  procurable. 

Sixth.  Reject  any  sticks  liiul  show  signs  of  worm-holes,  de- 
cay, scorching  by  forest  Arcs,  riug-lieart,  ring-shakes,  rotten 
or  black  knots,  dark  or  discoloie<l  spots,  or  any  other  (l(  feel 
that  would  impair  the  strength  or  durability  of  the  timber. 

Seventh.  Examine  carefully  l>y  probing  with  a  wire  jill 
hollow  or  bird-eye  knots,  and,  should  the  hollow  be  over  one 
inch  deep,  reject  the  limber. 

Eightli.  Check  lengths  of  cutting  gauges  every  day,  as  they 
are  liable  occasionally  to  be  knocked  out  of  position.  Ciieck 
widths  and  thicknes.ses  at  each  change  of  the  machine. 

Ninth.  In  inspecting  i)iles,  look  carefully  to  their  straight- 
ness,  and  see  that  they  comply  in  this  and  in  every  other  par- 
ticular with  tiie  si>eciflcations. 

Tenth.  See  that  due  care  is  used  in  handling  and  loading 
timber  so  as  not  to  bruise  it ;  and  under  no  consideration  allow 
it  to  be  floated  in  the  water  after  it  is  cut  and  dressed. 

Eleventli.  Keep  a  daily  record  of  all  timl)er  accepted,  .so 
that  the  Engineer  may  be  informed  on  short  notice  as  to  how 
much  of  any  bill  has  been  cut. 

Twelfth.  Notify  the  Engineer  or  other  proper  party  of  all 
shipments,  and  keep  an  accurate  account  of  everything 
shipped,  so  that  upon  shojt  notice  a  statement  in  respect  to 
any  uncompleted  order  can  be  made,  giving  the  amount  that 
has  been  shipped  aud  the  amount  that  reuidins  to  be  for- 
warded. 

Thirteenth.  Tlie  Cliief  Inspector  must  make  regular  monthly 
reports  to  the  Engineer  or  other  proper  party  or  parties  con- 
cerning the  progress  of  the  work,  (pnilily  of  timber  furnished, 
etc.;  and  must  send  in  montiily  statements  of  all   moneys 


300 


t)E  Po>rntn:s. 


received  and  expended  by  him  in  couneclion  with  his  work 
of  inspection. 

Fourteentli.  Use  every  endeavor  not  to  cause  by  your  in- 
spection iuiy  more  biiudling  of  material  than  is  necessary  for 
doing  your  work  thoroughly  ;  and  do  nothing  to  give  the  mill 
people  needless  wo*  ry  or  expense. 

In  concluding  th  s  chapter,  the  author  desires  to  emphasize 
his  previous  stateiaent  that,  in  order  to  obtain  a  truly  tirst- 
class  structure,  it  is  necessary  not  only  to  design  it  properly 
and  prepare  thorough  specifications  for  its  building,  but  also 
to  provide  a  corps  of  competent  and  honest  inspectors,  who 
will  from  start  to  finish  examine  carefully  and  test  all  mate- 
rials that  are  to  be  used,  and  who  will  see  that  the  entire 
manufacture  and  erection  are  done  in  strict  compliance  with 
the  specifications. 


CHAPTER  XXII. 


DESIGNING    OF    PIERS. 


The  object  of  tlii.s  chapter  is  not  to  provide  the  bridge- 
builder  with  either  a  complete  specifiailioii  for  building  piers 
of  all  kinds  or  full  directions  as  to  sinking  them  under  nil 
possible  circumstances,  but  to  indicate  to  the  designer,  tirst, 
how  to  determine  the  best  kind  of  piers  to  use  at  any  pro- 
posed crossing,  and.  second,  how  to  proportion  them.  Text- 
books on  substructure  do  not  generally  cover  tiiis  ground, 
but  deal  mainly  with  masonry  specifications  and  methods  of 
sinking  piers.  The  reader  who  desires  to  learn  anything  con- 
cerning piers  which  is  not  given  in  this  chapter  is  referred  to 
Baker's  "  Treatise  on  M:isonry  Construction  "  and  Pattou's 
"  Practical  Treatise  on  Foundations." 

In  determining  the  layout  of  spans  for  any  important  cross- 
ing, the  first  (juestion  to  settle  is  what  method  of  pier-sinking 
to  adopt,  for  upon  this  will  depend  to  a  certain  extent  the 
span  lengths. 

The  three  principal  methods  in  common  use  are  as  follows  : 

1.  The  Coffer-dam  system. 

2.  The  Pneumatic  j^roce-ss. 

3.  The  Open-dredging  process. 

The  use  of  coffer-dams  is,  or  should  be,  limited  to  crossings 
where  the  bed  rock  is  not  more  than  fifteen  feet  below  the 
ordinary  stage  of  water,  and  where  there  is  no  great,  sudden 
rise  anticipated.  This  method  always  figures  low  in  the  pre- 
liminary estiiiiate,  but  is  generally  found  to  run  much  higher 
when  the  total  cost  of  the  finished  structure  is  co:npute(l. 
The  author  nearly  always  discourages  his  contractors  from 
attempting  to  use  this  jnethod  ;  and  thus  far  l>is  experience 

FROVhNC'^L    LIORARY 
VICTORIA,  B.  C. 


305; 


DE    rONTIHUS. 


proves  that,  when  they  fiiil  to  adopt  iiis  advice  about  it,  I  hey 
are  generally  sorry  therefor  by  the  time  tiie  work  is  finished. 

Coffer-dams  are  liable  to  give  trouble  in  several  ways:  first 
b}'  leakai^c,  second  by  flooding,  and  tiiird  by  collapsing.  If  a 
Contractor  gets  tliroiigh  a  large  piece  of  (!ofTer-dam  foundation 
work  witiioul  accident  or  trouble  of  SiUie  kind  he  is  in  great 
luck. 

For  bare  bed-rock,  movable  coffer-dams  may  be  emjiloyed  ; 
but  they  are  troublesome  to  con.slruct,  and  are  sometimes  very 
(lifHcult  to  remove  because  oi  a  deposit  of  sand  taking  place 
while  the  piers  are  being  built. 

Tiie  pneumatic  process  for  sinking  piers  is  in  most  cases  the 
best  one  to  employ,  the  only  objection  to  it  being  the  exces- 
sive cost  of  installing  the  plant,  even  if  one  has  a  complete 
outfit  at  his  disposal.  Its  great  advantages  are  that  it  enables 
the  contractor  to  overcome,  in  thecheipest  and  most  expeditious 
manner  possible,  all  obstacles  that  may  be  encountered  in 
sinking;  and  that  it  ensures  the  obtaining  of  a  satisfactory 
foundation  for  the  piers.  It  can  be  used  for  depths  as  great 
as  one  hundred  feet  or  even  more,  although  there  is  consider- 
able danger  to  the  workmen  when  the  depth  exceeds  eighty  or 
ninety  feet. 

Most  of  the  bridge-piers  which  the  author  has  put  in  have 
been  sunk  by  the  pneumatic  process  ;  and  he  has  no  hesitatio!i 
in  recommending  it  as  the  most  satisfactory,  all-around  method 
in  probably  nine  cases  out  of  ten  which  occur  in  a  consuliing 
engineer's  practice. 

The  open-dredging  process  is  suitable  for  very  deep  founda- 
tions, or  for  putting  down  caissons  that  are  to  rest  on  the  sand, 
or  for  bed-rock  foundations  where  there  is  no  liability  of  great 
scour.  For  large  piers  this  process  is  much  cheaper  tlian  the 
pneumatic  on  account  of  both  the  smaller  cost  of  i)lant  and  the 
more  rapid  progress  in  sinking.  In  case,  however,  that  ob- 
stacles be  encountered,  such  as  trees  or  large  boulders,  the 
expense  for  sinking  is  liable  to  run  high,  as  these  obstacles 
may  have  to  be  removed  by  a  diver  or  divers,  which  always 
involves  great  expen.se.  The  author  has  piit  down  three  large 
piers  by  the  open-dredging  jn'occss,  two  to  u  depth  of  uinetv 


nnsioNiNo  OF  piers. 


303 


/ 


f(H't  iind  one  to  a  dcptli  of  one  hundred  and  twenty- two  feet 
below  extreme  low  wnter,  and  lian  encountered  no  trouble 
worth  mentioning  during  the  sinking  In  the  case  of  the 
greater  depth,  a  mass  of  bouklers  was  found  overlying  the 
bed-rock.  This  was  penetrated  as  far  as  practicable  by  ex- 
cavating the  boulders  and  laying  bare  the  bed-rock  near  the 
centre  of  tiie  pier,  then  tiring  charges  of  dynamite  iil  the  bot- 
tom till  the  cylinder  refused  to  sink  any  farther,  after  whidi 
it  was  tilled  up  with  concrete. 

It  is  probable  tliat,  if  one  were  to  try  to  sink  small  pi«.rs  to 
any  great  depth  by  the  open-dredging  process,  difflcully  would 
be  experienced  because  of  the  lack  of  sufficient  weight  of 
pier  as  compared  with  the  large  amount  of  skin  friction.  The 
latter  in  sand  is  generally  a  little  less  than  six  hundred  pounds 
per  square  foot  of  vertical  surface.  On  the  East  Onudia 
bridge  the  author  arranged  to  reduce  this  friction  by  means 
of  small  wjiter-jets  placed  around  the  circumference  of  the 
cylinder  about  every  six  feet  in  height  ;  but  these  were  found 
to  be  unnecessary,  so  were  not  used.  In  case  of  striking  clny 
or  any  other  sticky  substance,  such  an  attachment  might 
prove  of  great  service. 

The  open-dredging  process  is  liable  to  abuse  by  the  builders 
of  cheap  highway  bridges,  who,  in  order  to  save  a  little  in 
first  cost,  use  it  to  sink  cylinder  piers  of  small  diameter  moder- 
ate distances  to  bed-rock,  which  may  in  these  i)l:i(es  be  laid 
baie  or  nearly  so  by  excessive  scour.  With  this  [n-ocess  it  is 
generally  not  practicable  to  anchor  the  cylinders  tirndy  to  the 
bed  rock,  but  with  the  pneumatic  process  it  is. 

There  is  still  anoliier  style  of  foundation  besides  the  three 
described,  viz.,  that  which  involves  the  use  of  piles.  These 
piles  may  either  support  a  timber  grillage,  upon  whieli  to  rest 
the  pier  proper,  or  may  run  up  into  the  concrete  body  of  the 
pier.  This  class  of  foundation  is  of  a  cheap  or<ler,  but  will 
often  answer  the  purpose  V(  ry  well,  provided  Iheie  be  no  jws- 
sibility  of  excessive  scour.  If  the  bearing  capacity  of  the 
piles  be  snndl,  it  is  best  to  spread  them  out  and  cover  them 
with  a  thick  timber  grillage  ;  but  otherwise  it  will  be  found 
pconoinical  to  run  the  piles  uj)  into  the  concrete.     The  author 


304 


DE    PONTIBUS. 


lias  lately  designed  the  piers  for  three  iinportftnt  Soulherii 
bridges  in  the  hitler  miuiner.  They  were  put  down  without 
much  (lifHculty,  the  principal  hindnince  being  from  sunken 
logs,  and  were  eminently  economicnl  in  first  cost  as  compared 
with  piers  of  other  possible  designs. 

Piers  may  be  divided  into  the  following  classes  in  respect  to 
tiie  nniteriids  of  which  they  are  built : 

1.  Stone-masonry  piers  resting  on — 

A.  Bed-rock. 

B.  Timber  and  concrete  caissons. 

C.  Steel  and  concrete  caissons. 

D.  'limber  giilhiges  supported  on  pile.s. 

2.  Brick-masonry  i)iers  resting  on  the  stime  foundations  as 
mentioned  for  slone- masonry  piers. 

3.  Unprotected  concrete  pieis  resting  on  the  same  foun- 
dations an  mentioned  for  stone-masonry  piers. 

4.  Oblong  steel  shells  filled  witii  concrete  and  resting  on 
the  same  foundations  as  mentioned  for  stone-masonry 
piers. 

5.  Cylinders  filled  with  concrete  and  resting  on — 

A.  Bed-rock. 

B.  Timber  and  concrete  caissons. 

C.  Timber  grillages  supported  on  piles. 

D.  Piles. 

6.  Braced  steel  piers  resting  on — 
A.  Bed-rock. 

B.'  Masonry,  brick,  or  concrete  piers. 
C.    Cylinder  piers. 

7.  Timber  piers  resting  on — 

A.  Mudsills. 

B.  Piles. 

Now  in  respect  to  which  of  these  seven  kinds  of  piers  and 
which  of  their  various  supports  it  is  best  to  adopt  for  any  par- 
ticular crossing,  the  engineer  must  use  his  judgment,  which, 
however,  may  be  aided  by  the  following  remarks  that  are 
bftsed  upon  thp  author's  experience  ; 


DESIGN  I  N(J    OF    I'IKKS. 


305 


Class  No.  1. 

Masonry  piers  should  be  used  for  important  railroad  bridges 
and  for  very  large  liigiiway  bridges,  where  lirst-class  stone  can 
be  obtained  at  reasonable  cost.  If  good  stone  is  not  c  Uiiimblc 
at  a  fair  price,  it  is  better  to  use  one  of  the  other  classes.  Tlit 
proper  way  lo  proportion  a  masonry  pier  is  to  determine  the 
least  size  under  coping  to  support  either  the  pedestals  them- 
selves or  the  pedestal-blocks,  as  the  case  may  be,  leaving  a 
small  margin  on  the  e.vterior  and  ample  room  between  ped- 
estals or  pedestal-blocks  to  allow  for  variation  in  erection, 
then  batter  the  pier  all  around  not  less  than  one-half  incli  to 
the  foot,  or  as  much  more  as  investigation  shows  to  be  neces- 
sary.  Tlie  coping  should  project  all  around  about  six  inches, 
the  amount  depending  upon  the  magnitude  of  the  pier  and 
the  thickness  of  the  coping  course,  which  should  be  from 
eighteen  to  twenty-four  inches. 

The  batter  for  tlie  sides  is  to  be  determined  in  the  following 
manner :  Compute  for  both  the  loaded  and  the  empty  struc- 
ture the  greatest  longitudinal  components  of  the  total  wind- 
pressure  that  can  come  upon  the  pier  from  tlie  two  spans 
which  rest  thereon,  upon  the  assumption  that  the  friction  at 
the  roller  ends  of  the  spans  is  zero. 

The  direction  of  the  wind  which  will  give  the  greatest 
longitudinal  thrust  on  the  structure  is  forfy-tive  degrees  in 
respect  to  its  longitudinal  axis.  As  the  cosine  of  this  angle  is 
approximately  seven  tenths,  the  longitudinal  component  of 
the  wind-pressure  per  lineal  foot  of  span  will  be  seventy  per 
cent  of  the  assumed  said  wind -pressure. 

Find  also  the  greatest  traction  thrusts  from  braked  trains  on 
the  assumptions  that,  first,  the  greatest  live  load  is  on  the 
structure,  and,  second,  that  the  least  live  load,  or  one  thousand 
pounds  per  lineal  foot,  is  on  same.  Now  find  the  values  of 
the  following  combinations : 

1.  Thrust  from  wind  load  on  empty  bridge, 

2.  Thrust  from  heaviest  braked  train. 

3.  Thrust  from  wind  load  with  lightest  live  load  on  the 
spans. 


;{()(> 


I»K    I'ONTIHUH. 


4.  Coinbiiied  lliiusi  from  lightest  possllilc  limkcd  train,  mid 
ft  wiiidpressure  ou  tiuiii  uud  structure  fiitiul  to  one  half  of 
llmt  8pecilied. 

Next  delenniue  by  judgment  the  propi;r  batter,  and  hiy  olT 
the  pier  to  scale;  then  divide  it  by  horizontal  planes  from  four 
to  six  feet  apart,  and  comii/.e  tlie  weights  of  iil!  the  |)ortion8 
of  the  masonry  between  hese  planes,  making  a  proper  reduc- 
tiou  for  weight  of  water  for  those  parts  which  would  be  sub- 
merged by  an  average  stage  of  river. 

Next  compute  the  wind-pnssure  on  each  vertical  division  of 
the  pier,  down  to  the  assumed  stage  of  water,  in  a  direction 
parallel  to  the  spans,  using  the  same  intensity  and  direction 
for  the  wind-pressure  us  were  adopted  in  finding  the  longi- 
tudinal thrust  from  wind-iiressure  on  the  spans. 

Next  lind  graphicidly  for  all  four  cases  the  curves  of  pres- 
sure from  the  vertical  and  horizontal  loads  at  top  of  pier,  com- 
bined with  the  weights  of  the  various  divisions  of  the  latter 
and  the  wind-pressures  thereon,  and  see  that  none  of  the  said 
curves  at  auy  plane  of  division  pass  outside  of  the  middle  third 
of  the  section  at  said  i)lane.  If  any  of  tliem  do,  the  batter 
will  have  to  be  increased,  or,  if  all  the  curves  fall  mucii  inside 
of  the  middle  third  points,  it  will  have  to  be  decreased;  and 
in  eitiier  case  the  graphical  computations  will  have  to  be  made 
again,  and  so  on  until  a  satisfactory  batter  is  found. 

The  author  is  aware  of  tlie  fact  that  this  method  of  design- 
ing piers  is  not  in  general  use,  and  it  is  (juite  possible  that  he 
is  the  sole  engineer  who  adopts  it  ;  nevertheless  he  maintains 
that  it  is  the  only  proper  way  to  design  masonry  piers.  The 
single  concession  wliich  he  would  be  willing  to  make  on  the 
.score  of  economy  would  be  to  assume  that  a  certain  small 
portion  of  the  thrust  ou  a  span  is  taken  up  at  the  roller  end. 
But  if  the  rollers  are  in  good  working  order  the  amount  of 
thrust  that  they  will  resist  is  very  small  indeed — so  small,  in 
fact,  that  the  author  prefers  to  neglect  it  entirely. 

The  ordinary  method  of  i)r()portioning  piers  is  to  make  them 
as  small  as  possible  under  coping  and  baiter  them  all  around, 
or  at  least  on  the  sides,  one-half  inch  to  the  foot.  In  some 
cases  this  will  suffice,  but  in  others  it  will  not.     One  of  the 


largest 
insuillci 
untraini 
these  pi 
omy  in 
properlj 
An  in 
general) 
superstri 
properly 
lever  bri 
out  four 
inch  to 
author  t 
pieis  to 
nppearan 
him,  and 
inches  to 
tory  appe 
In  neaj 
stream,  ( 
I  lie    proj 
strength 
sure.     A 
of  piers  i 
Construe 
enough 
resist  pro 
combinat 
Neverth* 
high  piei 
■wliere  th( 
as  a  mat 
overlurni 
length  of 
neatest  v 
cocked-hi 
Where 


DKSKJNINO    OF    IMKKS. 


3or 


\: 


litrgcst  bridges  in  tbc  Uiiilcd  Sliitcs  lius  piers  l)uilt  witli  sucli 
insullicieiit  biitlcr  tlial  il  is  evident  ut  n  gluQce,  to  eveu  i;ii 
uiitruined  eye,  timt  something  is  wrong.  By  tlie  way,  one  of 
these  piers  is  cniclied  from  lop  t«^  l)ott()ni.  owing  to  false  econ- 
omy iu  the  design,  but  not  lieeause  of  its  failure  to  /Igure 
properly  for  the  curve  of  pressure. 

An  inherent  sense  of  fitness  in  the  mind  of  the  designer  will 
generally  lell  him,  when  Ijo  looks  at  a  scale-drawing  of  the 
superstructure  and  piers  of  a  bridge,  whether  the  latter  are 
properly  proportioned.  In  the  case  of  the  :  ,cd  liock  canli 
lever  bridge  over  the  Colorado  Kiver  the  piers  were  first  laid 
out  fourteen  feet  wide  under  coping,  witli  a  batter  of  half  an 
inch  to  the  foot,  and  the  drawings  were  submitted  to  the 
author  for  his  criticism.  He  immediately  pronounceil  the 
piers  to  be  proportioned  incorrectly,  simply  because  of  their 
appearance.  Their  proportioning  was  then  turned  over  to 
him,  and  lie  found  by  trial  that  a  batter  of  one  and  a  quarter 
inches  to  the  foot  was  necessary.  This  batter  gave  a  satisfac- 
tory appearance  to  the  entire  layout. 

In  nearly  every  case  the  leugtli  of  the  piers  up  and  down 
stream,  determined  by  the  minimum  size  under  coping  and 
the  proper  side-batter  for  thrust,  will  provide  sufficient 
strength  and  stability  to  resist  both  current  and  wind  pres 
sure.  A  thorough  investigation  of  resistance  to  overturning 
of  piers  down-stream  is  given  iu  Baker's  "  Treatise  on  Masonry 
Construction."  In  il  he  proves  that  any  pier  which  is  large 
enough  under  coping,  and  which  has  ordinary  batter,  will 
resist  properly  the  overturning  tendency  of  the  worst  possible 
combination  of  loads  from  wind,  current,  and  floating  ice. 
Nevertheless,  in  long-span,  single-track  bridges  with  very 
high  piers,  crossing  swift  streams  that  carry  thick  ice,  and 
■where  the  structure  is  exposed  to  high  winds,  it  is  advisalile, 
as  a  matter  of  precaution,  to  test  the  piers  for  down-ptream 
overturning  according  io  Prof,  leaker's  method.  Should  the 
length  of  pier  parallel  to  the  stream  be  found  insufflcient,  the 
neatest  way  to  obtain  the  requisite  stability  is  to  put  in  a 
cocked-hat  just  above  the  elevation  of  extreme  high  water. 

Wljere  a  masonry  pier  rests  on  bed-ro<jk,  the  latter  should 


308 


DE    PON  TIB  US. 


be  levelled  or  slipped  off,  and  Ibere  should  be  placed  a  layer 
of  rich  coucrete  between  the  rock  and  the  masonry. 

If  an  ice-break  with  an  inclined  cutting  edge  be  necessary 
for  any  pier  that  rests  on  a  yielding  foundation,  a  correspond- 
ing ice-break  or  similar  offset  should  be  placed  at  the  down- 
stream end  of  the  pier  also,  even  if  its  appearance  be  as  incon- 
gruous as  would  thai  of  a  cowcatcher  al  Ihe  rear  of  a  lailway 
train  ;  for,  unless  the  foundation  be  thus  balanced  about  the 
centre  of  gravity  of  the  vertical  load,  the  portion  directly  under 
the  superstructure  will  lend  to  settle  njore  than  tiuil  under 
lb  >  nose,  and  will  thus  cause  a  cracking  of  the  masonry  and  a 
uplitling-off  of  the  front  of  Ihe  pier.  Such  a  disastrous  result 
of  the  violation  of  llie  principle  of  symmetry  in  designing  is 
by  no  means  unknown,  even  in  important  railroad  bridges. 

In  respect  to  timber-and-concrete  caissons  for  masonry 
piers,  the  following  general  remarks  will  prove  useful  to  the 
designer  : 

There  should  be  an  offset  of  not  less  than  two  feet  all  around 
the  base  of  the  masonry,  and  preferably  a  little  more  at  the 
ends,  so  that  in  case  the  caisson  be  located  a  little  out  of 
place  the  masonry  can  be  shifted  thereon  so  as  to  bring  the 
pier  into  proper  position.  The  number  of  courses  of 
13"  X  12"  timber  in  the  roof  of  the  caisson  should  never 
be  less  than  four  and  seldom  more  than  eiglit.  Any  less 
number  than  four  would  be  liable  not  to  give  the  roof 
the  proper  stiffness  during  the  sinking,  and  any  more 
than  eight  would  tend  to  cause  an  undue  settlement  of  the 
l)ier  on  account  of  the  compression  of  the  timber,  which 
always  takes  place.  The  designins;  of  the  roof  and  sides  of 
I  lie  working-chamber  should  bo  lone  with  the  greatest  care, 
so  as  to  preveut  all  possibility  of  collapse,  and  tlie  cutting 
edge  should  be  shod  with  steel  plates  to  protect  the  timber 
when  the  caisson  is  piussing  through  bouldirs  or  logs.  Tiie 
roof-timbers,  if  possible,  should  always  be  of  full  length,  and 
the  spacing  of  the  bolls  therein  should  not  e.\ceed  four  feet. 
The  vertical  timbers  on  the  outside  of  the  working-cliamber 
should  be  carried  well  up  into  the  roof,  shoulderi  d,  and  firmly 
bolted  thereto.     The  crib  above  ti.'!  working-chamber  should 


DESIGNING   OF   PIERS. 


309 


be  sbeiitlied  so  as  to  reduce  the  friction  duriug  siuking.  The 
drift-bolts  should  be  seveii-eigliths-mch  rouuds  driveu  into 
three-quarter-iiicli  bored  holes.  The  tilling  of  the  working- 
chamber  with  concrete  should  always  be  done  with  the  great- 
est care,  using  extra  ricli  concrete,  so  that  there  shall  be  no 
voids  between  the  concrete  and  the  roof.  Portland-cement 
of  the  best  quality  should  be  used  for  filling  the  working- 
chamber  and  shafts ;  but  it  is  legitimate  to  employ  an  extra- 
good  q\uiliiy  of  Americau  natural  cement  for  filling  the  crib 
in  case  that  it  be  necessary  to  keep  down  the  expense.  How- 
eve:',  Portland  cement  is  always  prefera'>Ie. 

In  respect  to  caissons  built  of  steel  and  concrete  but  little 
need  be  said,  except  that  great  care  should  be  taken  to  design 
the  working-chamber  strong  enough  to  resist  properly  the 
weight  of  the  concrete  above  and  the  unequal  pressures  from 
boulders  below.  The  metal  below  the  roof  of  the  working- 
chamber  should  not  be  less  than  one-half  inch  in  thickness, 
and  all  parts  near  the  cutting  edge  should  have  thicknesses 
varying  from  three  (pinrters  of  an  inch  to  an  inch.  All  joints 
in  the  cutting  edge  siiould  be  full  spliced,  as  should  also 
those  in  the  roof  of  the  working-chamber. 

Timi»er  grilliig<'s  resting  on  piles  should  have,  preferably, 
not  less  than  four  courses  of  timber,  although  often  but  three 
and  more  rarely  two  are  emp'oyed.  As  the  grillage  is  gener- 
ally wider  than  the  masonry,  it  takes  about  four  courses  to 
distribute  the  weight  uniforndy  (or  nearly  uniformly)  over  the 
piles.  In  case  of  an  unusually  wide  grillage,  more  than  four 
courses  would  be  necessary,  or  else  the  masonry  should  be 
widened  by  means  of  footing-courses.  Such  grillages  should 
be  built  with  care,  so  as  to  have  a  level  bottom;  and  all  piles 
should  be  cut  oft  to  exact  level,  otherwise  there  will  be  un- 
equal bearing  between  piles  and  grillage  that  might  cause 
serious  damage  to  the  masonry. 

Brick  piers  are  not  common  in  America,  probably  because, 
until  lately,  it  has  been  difil  ilt  to  obtain  jiroper  brick.  In 
the  author's  opinion,  piers  built  exclusively  of  hard-burned 
clinker  brick  and  mortar  of  the  very  best  quiilily  of  Portland 
cement,   nnxetl   in   the   proportion   of    one    i)art    cenumt  to 


310 


DK    PON  Tift  US. 


two  parts  sand,  and  having  tliiu  joints  perfectly  filled,  are 
better  than  the  average  masonry  pier,  for  the  reason  that  tlie 
bricks  will  never  disiulegrale,  while  the  average  stone  vised 
for  bridge-piers  will.  The  author  lias  not  yet  had  occasion 
to  build  any  brick  piers;  but  he  intends  to  give  tiiem  a  trial 
on  the  first  opportunity. 

Unprotected  '-one  -h  piers  are  satisfactory  for  Southern 
rivers,  where  tic  ''<  •  '  frost  is  not  severe,  and  where  there 
is  no  ice  of  any  juiioiuit  carried  by  the  stream.  The  author 
used  this  style  of  pier  for  the  Arkansas  Kiver  bridge  of  the 
Kansas  City,  Pittsburg,  and  Gulf  Railroad  near  Redland,  Ind, 
Ter.  SevLM-al  othtr  Southern  bridf^es  iiave  piers  of  this  type 
and  Ihps  lar  they  have  proved  satisfactory,  Tlieir  chief  reccni- 
mendation  is  their  cheapness.  In  order  to  ensure  their  being 
properly  built,  nothing  but  the  best  qualities  of  cement  and 
sand  should  be  employed,  and  the  mortar  for  the  concrete 
should  be  mixed  rich,  cspeciMlly  uv.m  the  exterior  of  the  piers. 
Some  engineers  give  the  work  a  skin-coating  of  ri'jh  mortiir; 
l)Ul  the  author  prefers  to  use  fuuly  l)r()ken  stone  and  extra- 
rich  mortar  for  six  or  eight  iiiches  all  around,  and  to  not  at- 
tempt to  smooth  down  the  •;>(  Mor.  Of  course  it  is  practi- 
cable to  put  on  a  .skin-cou's  >  f'.o  U  will  stay,  and  M)  that  it 
will  not  have  a  streaky  !i ;■;■,  r;,ic  ■  but  to  do  this  requires 
more  fare  tiian  liie  average  wo;  .r  .jm  is  inclined  to  take. 

Steel  shells  filled  witli  concr<'le  ;.!.i.ue  very  satisfactor}' piers, 
provided  they  be  not  used  in  salt  or  brackish  water,  wliich 
would  rust  them  out  in  a  short,  time.  These  piers  are  appli- 
cable where  good  stone  for  masonry  is  <  ^pensive,  and  where 
the  piers  must  be  protected  from  the  abrasion  of  ice  or  from 
the  excessive  »old,  which  would  tend  to  disintegrate  even 
fairly  good  concrete.  Such  piers  can  l)e  built  in  the  usual 
form  of  masonry  piers  ^  ■';  rounded  ends  all  the  way  up,  or. 
when  they  pass  much  u-^^'  high  water,  they  may  run  olT  into 
two  cylinders  with  bracin.,;  urt-veen.  Butt-sp!  ces  are  prefer- 
able, and  the; , .lice-plates  below  the  mud  linoshould  be  placed 
on  the  in^.de  soss  to  oiler  as  little  resistance  as  possil)le  to  sink- 
ing. 

This  style  of  pier  is  a  favorite  one  of  the  author's,  for  the 


DESIGNING    OP    PIERS. 


31i 


reason  tliat  it  is  both  sightly  and  inexpensive.  When  taken 
to  task  for  using  it,  as  often  hapi)ens,  lie  replies,  "  Good  con- 
crete protected  with  steel  is  better  than  poor  masonry." 

In  respect  to  the  thickness  of  stiiel  to  use,  the  author's  prac- 
tice is  to  adopt  lialf  an  inch  below  tiio  ordinary  stage  of  water 
and  tliree  eighths  of  an  inch  above,  although  for  cheap  bridges 
he  occasionally  shades  these  thicknesses  one  sixteenth  of  an 
inch. 

For  the  coping  of  such  piers  stone  may  be  employed  ;  but 
it  is  preferable  to  put  on  a  moulding  of  siieet  metal,  as  this  is 
more  in  keeping  with  the  vest  of  the  pier.  This  .style  of  cop- 
ing has  been  criticised  on  the  plea  that  it  is  false,  and  that  it 
has  no  direct  function  ;  nevertheless,  the  author  considers  it 
eminently  proper  to  use  it,  and  that  us  function  is  .simply  to 
beautify  the  construction  by  relieving  the  harsh  outlines. 
Where  stone  coping  is  not  used,  the  top  of  any  kind  of  con- 
crete pier  may  be  finished  off  with  either  ricli  concrete  of 
small  broken  stone  or  with  granitoid,  mixed  in  tlie  proportion 
of  one  part  of  Portland  cement,  two  parts  tine  granite  screen- 
ings, :ind  three  parts  of  small  cr>isiied  granite. 

Cylinder  piers  filled  with  concrete  are  the  most  common 
kind  of  pier  in  America,  and  they  are  certainly  the  worst; 
nevertheless  they  have  their  i)lace  in  good  construction* 
when  they  are  jiroperly  designed  and  binlt.  Their  abuse  is 
due  mainly  to  tiie  builders  of  cheap  highway  bridges,  who 
think  that  if  the  top  of  the  cylinder  is  simply  large  enough  to 
hold  the  pedestals,  tiiatisall  which  is  necessary,  no  matter  how 
high  the  piers  may  be,  how  great  may  be  the  scour,  or  what 
kind  of  foundati(m  there  is.  If  piles  are  employed  as  a  foun- 
dation, they  put  in  all  tliat  their  small  cylinders  will  hold,  and 
never  dream  of  its  being  necessary  to  figure  how  many  tons 
each  pile  will  have  to  sustain. 

Cylinder  piers  are  legitimate  construction  in  places  where, 
under  the  worst  possible  conditions  in  respect  to  scour,  they 
will  have  a  firm  grip  in  solid  material,  say  not  less  in  depth 
than  twenty  per  cent  of  the  height  of  the  entire  pier. 

Cylinder  piers  will  iK.t  often  stand  tiie  test  of  the  curve  of 
p    jsures  herein  (h-scribt'd  for  masonry  piers  ;  but  this  is  not 


312 


DE   PONTinUS. 


accessary,  because  they  can  resist  tension  on  one  side  in  both 
the  metal  ami  the  couciele,  if  the  latter  be  of  the  correct  qual- 
ity ;  i.e.,  the  cylinders  can  act  as  beams  to  resist  the  horizontal 
thrust  of  wind  and  trains  in  the  same  way  as  do  the  columns 
of  elevate<l  railroads.  Nevertheless  for  railroad  bridges  the 
author  would  advise  against  the  adoption  of  long  cylinders 
for  piers,  on  account  of  their  inability  to  resist  vibration  ef- 
fectively. In  some  cases  it  is  eccmomical  to  adopt  a  group  of 
four  comparatively  small  cylindeis  well  braced  on  all  four 
faces ;  but  with  this  style  t)f  foundation  it  is  generally  cus- 
tomary to  employ  braced  |)ier8  resting  on  tlie  cylinders. 

The  diameter  for  a  cylinder  should  depend  not  only  upon 
the  size  required  at  the  lop,  but  also  upon  its  height  and  the 
character  of  the  foundation.  It  issometimes  governed  also  by 
the  total  vertical  load  lol).;  carricil,  which  should  under  no  cir- 
cumstances exceed  the  limit  set  in  the  spocificalions  given  in 
Chapter  XIV.  Portland  cement  only,  and  that  of  the  very 
best  quality,  sliould  be  used  for  tilling  cylinder  pieis,  and  the 
filling  should  be  done  wiih  the  greatest  care  and  thoroughness. 
Whenever  the  concrete  has  to  be  placed  below  water  it  should 
be  done  by  using  a  tremie,  and  the  composition  of  the  con 
Crete  should  be  much  richer  than  that  for  concrete  laid  in  the 
dry. 

Whenever  a  cylinder  is  sunk  to  bed-rock,  it  should  be  let 
into  same  far  enough  to  prevent  all  possibility  of  slipping,  and 
so  as  to  give  an  even  bearing  all  anmnd  the  circuniference. 
This  is  an  easy  matter  when  the  pneumatic  process  is  employed 
forsinking,  but  it  is  often  difficult  when  open  dredging  is  used. 
This  i^recaulion  is  as  necessary  in  the  case  of  wooden  or  steel 
caissons  as  it  is  for  cylinder  piers,  and  should  never  be  neg- 
lected where  there  is  a  possibility  of  sco\ir  to  or  near  bed-rock, 
or  where  the  pneumatic  process  is  employed. 

In  sinking  large  cylinders  by  the  open  dredging  process  so 
as  to  fill  them  afterwards  with  piles,  it  sometimes  becomes 
Beces.sary  to  i)Ut  in  temporary  timber  bracing  bolted  to  the 
metal,  in  order  to  prevent  the  cylinders  from  collapsing  or 
from  getting  out  of  shape.  Most,  if  not  all,  of  these  timbers 
will  have  to  be  removed  before  the  piles  can  be  driven. 


DESIGNING  OP  PIERS. 


313 


It  is  ecouomicfil  soiuetiau's  lo  iuciease  the  diameter  of  a  cyl- 
iudcr  between  top  aiul  bottom,  but  in  suclj  cases  the  lower 
twenty  feet  should  be  made  plumb  so  that  the  cylinder  can  be 
sunk  with  case  and  accuracy.  This  detail  Wiis  adopted  for 
the  Jefferson  City  bridge,  the  variation  in  diiimcter  being  ob- 
tained by  telescoping  some  of  the  lengths  and  putting  in  fill- 
ing-rings. This  required  a  tritie  more  metal  than  truly  conical 
piers  would  ;  but  the  shop-work  was  much  simpler. 

The  proper  load  for  large  piles  inside  of  cylinders  is  about 
thirty  tons  each,  although  with  very  large  piles  and  solid  ma- 
terial lo  hold  them  it  may  lie  increiised  to  forty  tons.  On  the 
other  hand,  in  case  of  bad  foundations,  it  is  sometimes  neces- 
sary to  reduce  the  load  to  ten  tons  per  pile. 

The  proper  distance  for  piles  lo  project  into  cylinders  is 
about  fifteen  feet,  and  should  never  be  less  than  ten  feet  or 
more  than  twenty  feet  undi  r  any  circumstances.  With  less 
than  ten  feet  there  will  not  be  enough  grip  for  the  concrete, 
and  with  more  than  twenty  feet  it  is  dillieult  to  place  the  con- 
crete properly  under  water  between  the  piles.  Piles  in  cylin- 
ders should  be  driven  iis  closely  as  po.ssible,  and  precaution 
should  be  taken  to  jirevent  the  lifting  of  piles  already  driven 
by  the  sinking  of  the  last  few  piles,  Whei\  large  piles  are 
employed,  the  designer  can  figure  upon  six  scpiare  feet  of  pier 
section  for  each  pile. 

The  bracing  between  the  up-stream  and  the  downstream 
cylinders  of  a  pier  should  invariably  be  of  solid  webs  properly 
stiffened,  extending  from  high  water  to  near  low  water,  in 
case  there  be  any  drift ;  but  for  cylinder  piers  on  shore  an 
open  bracing  of  struts  and  lies  will  suffice. 

Concerning  braced  steel  piers  but  little  need  be  said,  except 
that  they  should  conform  in  their  design  with  the  specifica- 
tions given  in  Chapters  XIV  and  XVI.  It  is  advisable,  if 
practicable,  to  avoid  battering  more  than  two  faces  of  a  braced 
pier  on  accoimt  of  the  troublesome  shoi)-work  that  would  be 
involved  with  a  four-face  batter  ;  nevertheless  it  is  often  neces- 
sary to  adopt  the  latter,  especially  for  high  piers.  A  possible 
objection  to  this  type  of  pier  is  that  for  cantilever  bridges  it 
increases  the  defiection  of  the  span  because  of  the  compression 


814 


DE   PONTIBU!?. 


of  tlie  pier  columns  under  loud.     An  extreme  instance  of  such 
compression  is  that  in  the  Niagara  Cantilever  bridge. 

Timber  piers  are  merely  a  makeshift,  so  do  not  merit  much 
consideralion.  Tlieyaro  'employed  sometimes  to  support  steel 
l)ridges  until  mon'.y  is  available  for  building  masonry  piers. 
It  is  seldom  that  timber  piers  are  built  in  large  rivers  where 
the  current  is  rapid  and  the  scour  is  great.  The  author  was 
once  forced  by  circumstances  into  building  pile  piers  under 
these  conditions  ;  and  although  tliey  are  still  standing,  he 
would  sleep  belter  at  certain  seasons  of  the  year,  had  tiiey 
never  been  built.  The  piers  referred  to  are  the  temporary 
piers  of  the  East  Omaha  bridge  over  tlie  Missouri  River.  Tliey 
were  constructed  in  tlie  winter,  mostly  on  tlie  sand-l)ar,  by 
driving  groups  of  seventy-foot  red-cypress  piles  lifty  feet  into 
the  sand  by  means  of  a  powerful  water-jet,  then  sheathing 
the  sides  and  nose  with  four-inch  oak  pljinks  and  bracing  the 
piles  on  the  inside.  The  nose  of  each  pier  is  on  an  incline, 
faced  witli  steel  plates  where  the  ice  can  reach  il,  and  forming 
a  cutting  edge  that  is  capped  with  a  heavy  nulroad  rail.  Each 
pier  is  surrounded  with  a  woven  willow  mattress,  eighteen 
inches  thick,  of  the  most  .substantial  character,  sunk  and  kept 
in  place  witli  rock.  These  piers  have  received  a  much  more 
severe  test  than  was  anticipated  when  they  were  de.><igned,  be- 
cause the  channel  has  shifted  across  the  river,  so  that  at  times 
there  are  thirty-five  feet  of  water  where  there  was  a  dry  sand- 
bar when  the  bridge  was  constructed.  The  mattresses  have 
not  been  injured  by  the  .scour,  but  have  simply  been  lowered, 
the  edges  going  deeper  than  the  portions  near  the  piers.  Tlie 
only  ill  effect  noticable  is  the  springing  down-stream  of  the 
lops  of  two  piers,  in  one  case  about  six  inches  and  in  the  oilier 
about  eleven  inches.  In  order  to  bring  the  tops  of  these  piers 
partially  back  to  place  and  prevent  any  further  deflection,  the 
author  employed  a  detail  which  has  proved  lobe  very  satisfac- 
tory. It  consists  in  passing  one  end  of  a  stiong  iron  chain 
loosely  around  an  up-stnam  pile  and  dropping  the  loop  to  tiie 
bottom,  then  attaching  near  the  oilier  end  of  the  chain  a  stc>el 
rod  with  an  adjustment.  A  number  of  these  chains  were  used 
for  each  pier,  the  rods  passing  through  heavy  timbers  ut  the 


t 
a  I 


fo 
wi 
so 
ini 
de 

W( 


DESIGNING  OF   PIEliS. 


315 


rear  of  the  pier  near  the  top.  By  screwing  up  on  these  ad- 
justments the  tops  of  the  two  piers  were  moved  baclc  a  little. 
Provision  is  nmde  for  future  scour  by  leaving  some  spare  chain 
beyond  the  point  at  whicii  the  rod  takes  hold,  so  thai  one  chain 
at  a  time  Ciiu  be  loosened,  lowered,  and  retightencd.  These 
East  Omaha  bridge-piers  will  probably  la.st  a  long  time  yet, 
although  when  they  were  put  in  no  one  anticipated  that  they 
would  be  needed  for  more  than  eiifht  years. 

In  sinking  piers  the  greatest  care  should  always  be  taken  to 
start  them  in  e.\acl  position  and  to  keep  them  there.  The  in- 
stant it  becomes  evident  that  a  pier  or  cylinder  is  get'  i\s  out  of 
correct  position,  it  should  be  moved  back,  even  if  it  i^e  neces- 
sary to  slop  the  sinking  entirely  until  the  true  position  be  re- 
covered. Generally  it  is  feasible  to  build  a  frame  of  piles  and 
heavy  timbers  around  each  pier  or  cylinder,  so  as  to  guide  it 
to  exact  position  at  all  times,  barritig  a  slight  springing  of  the 
piles,  which,  however,  can  generally  be  guarded  against. 

Some  sixteen  years  ago  the  author  had  occasion  to  sink  four 
eight-foot  cylinders  in  the  Des  Moines  River  by  open  dredging 
to  bed- rock,  so  as  to  form  a  single  pier,  tlie  axes  of  the  cylin- 
ders being  located  on  the  corners  of  a  twenly-four-foot  square, 
irnfortunatcly  the  author  in  making  the  design,  owing  to  in- 
experience, had  provided  no  allowance  for  variation  of  location. 
The  foreman  of  construction  informed  him  that  it  was  abso- 
lutely impossible  to  sink  those  four  cylinders  so  correctly  that 
the  struts  would  lit  between  their  tops  ;  consequently  the 
author  was  compelled  to  undertake  the  superintendence  of  the 
work  him.self.  He  built  a  strong  frame  of  piles  and  heavy 
timbers,  all  thoroughly  braced,  around  the  space  to  be  occu- 
pied by  each  cylinder,  cutting  out  the  horizontal  timbers  to  tit 
the  curve  of  an  eight-foot  circle,  and  even  gouging  out  places 
for  the  rivet-heads  to  pass.  One  of  these  horizontal  guides 
was  located  close  to  the  surface  of  the  water,  and  the  other 
some  nine  or  ten  feet  higher.  The  cylinders  were  dropped 
into  these  guides  and  sunk  to  bed-rock.  After  all  four  cylin- 
ders were  in  place,  and  partly  iillt'd  with  concrete,  the  struts 
were  inserted  between  their  tops,  and  were  found  to  furnisli  a 
driving  fit.     This  resull  was,  perhaps,  due  as  much  to  good 


316 


DE  fOKTiBirS. 


luck  as  to  good  management ;  but  the  experience  taught  tho 
author  a  lesson  which  he  has  never  forgotten,  and  which  he 
desires  to  impress  upon  all  young  designers,  v./  that  in  pre- 
paring any  substructure  design  it  is  essentuil  to  provide 
liberally  for  all  possible  variations  from  correct  position  m  all 
parts  of  the  work.  ,, 

In  respect  to  the  designing  of  pedestals  for  elevated  la  1- 
louds  and  the  delerminati(,n  of  the  bearing  capacities  of  soils, 
the  reader  is  referred  to  the  author's  before-mentioned  paper  on 
Elevated  Railroads  published  iu  the  1897  Transactions  of  the 
American  Society  of  Civil  Engineers. 


CHAPTER  XXIII. 


TRIANOULATION. 


The  necessity  for  extreme  accuracy  in  the  tHangiilntion  for 
piers  of  long  briiiges  is  not  generally  recognized  ;  hence  result 
errors  in  pier  location  that  sometimes  riquire  the  lengthening 
or  shortening  of  the  superstructure,  or  which  involve  tlie 
adoption  of  an  unanticipated  skew.  There  is  no  excuse  wlial- 
8oever  for  any  such  errors  in  location,  because  the  method  of 
triangulution  adopted  should  provide  a  check  against  not  only 
blimders,  but  also  even  trifling  variations  from  correctness  of 
posiiion.  antl  because  the  Contractor  should  inviui!il)ly.  at  the 
outset  of  his  work,  talie  sncb  precautions  as  will  prevent  the 
occurrence  of  any  variation  in  sinking  in  excess  of  that  pro- 
vided for  in  the  Engineer's  plans. 

In  the  triangulations  for  bridges  over  large  rivers,  such  as 
the  Missouri,  the  author  makes  a  practice  of  measuring  each 
base-line  five  times  and  each  angle  thirty  times  ;  and  no  point 
Is  ever  located  without  using  a  check  from  another  base-line, 
thus  providing  an  intersection  of  three  lines,  which  theoreti- 
cally should  be  a  mathematical  point,  but  which  actually 
varies  therefrom,  generally  about  a  quarter  of  an  inch,  and 
sometimes  even  as  much  as  one  half  of  an  inch,  in  sights  of 
about  one  thousand  feet  length. 

The  author  has  tried  lioth  iron  rods  and  steel  tapes  for 
measuring  base-lines,  and  has  adopted  the  latter  as  the  more 
accurate.  The  objection  to  using  rods  is  that  it  is  almost 
impossible  to  run  a  line  a  thousand  feet  long  with  three  rods 
that  must  always  be  made  to  actually  touch  each  other  with- 
out sometimes  disturbing  slightly  the  position  of  two  of  the 
rods,  when  either  liftinj^'  or  putting  down  the  third  rod.   With 

317 


318 


DB   PONTIBUS. 


iv  reliable  steel  tape  properly  handled,  the  extreme  error  in  a 
iniinbcr  of  measurements  of  the  siinio  line  should  he  less  than 
one  quarter  of  an  inch  in  one  thousand  feet.  This  would 
make  the  probable  error  of  the  avenige  length  considerably 
less  'han  that  amount.  If  any  measurement  sliow  a  greater 
variation  from  the  average  than  one  quarter  of  an  inch  to  the 
thousand  feet,  it  siiould  l)e  rejected,  and  another  measurement 
should  be  maih;  to  replace  it.  This  presupposes  comparatively 
level  ground  for  tiie  base-line  ;  hence,  if  iho  ground  bo  very 
irregular,  a  greater  variation  may  be  allowed.  It  should, 
however,  in  no  case  exceed  one-half  inch  per  thousand  feet. 

Tiie  tape  line  used  should  he  a  new  one  for  each  structure, 
and  it  should  be  tested  at  the  bridge  shops  in  comparison  with 
their  standard.  As  a  matter  of  precaution,  it  is  well  to  test  it 
in  the  field  with  another  tape  that  is  to  be  set  aside  Jis  a  reserve 
and  not  used  unless  an  accident  happen  to  the  primary  tape. 

For  very  long  and  important  bridges,  especially  cantilevers 
with  ^ong  spans,  it  would  be  well  to  have  the  tape  tested  by 
the  Bureau  of  Weights  and  Measures  at  Washington,  D.  C, 
or  by  some  other  testing  bureau  of  recognized  standing — such, 
for  instance,  as  that  of  the  Washington  University  at  St.  Louis, 
Mo,     The  charge  for  such  testing  is  usually  merely  nominal. 

As  the  coefficient  of  expansion  is  not  the  same  for  all  tapes, 
it  might  be  advisable  for  extremely  accurate  work  to  have  the 
coefficient  determined  for  the  tape  to  be  used;  but  in  most 
cases  of  long-span  bridges  this  would  be  an  unnecessary  re- 
finement. 

A  fifty-foot  tape  is  long  enough,  and  is  in  many  respects 
preferable  to  those  of  greater  length.  The  author  has  no  use  for 
extremely  long  tapes  to  measure  distances  directly  between  pier 
centres  either  during  sinking  or  after  the  piers  are  finished,  be- 
cause this  method  of  measurement  is  by  no  means  as  accurate 
as  that  of  intersecting  three  lines  on  each  pier  and  using  two 
independently  measured  base-lines.  The  only  direct  measure- 
ment that  is  of  any  real  value,  and  which  can  be  obtained 
before  the  falsework  is  up,  is  one  made  on  the  ice.  In  such  a 
measurement  care  must  be  taken  not  to  let  the  tape  touch  the 
ice,  but  to  rest  it  on  plugs  driven  on  perfect  line  into  holes 


TUlAN(a  l.ATiON. 


319 


therein  mid  cut  off  to  exiict  level.  Tlicre  is  no  inoie  diJlicuU 
nioasureiiient  to  miike  correctly  than  oue  with  m  long  steel  tape 
between  two  (listunt  jMjints  without  intermediate  supports;  he- 
cause,  in  tho  tirst  place,  the  doultlo  nieusurcnuiiit  on  shore  and 
in  correct  position  is  a  slow  and  tedious  one  to  niuke,  involv- 
ib^  us  it  does  the  use  of  the  level  to  obtain  the  sig,  which 
must  be  exactly  alike  in  l)oth  cases,  and,  in  the  second  place, 
the  conditions  of  wind  and  temperature  are  likely  to  ynry  to 
such  an  extent  as  to  cause  errors  that  are  very  difficult  to 
correct. 

All  base-line  measurements  should  be  made  in  cloudy 
weather,  or  just  after  sunset,  or  even  at  uight;  and  the  tem- 
perature should  be  noted  for  each  lifty  fiel  measured,  as  all 
lengths  must  be  reduced  to  those  for  an  assumed  standard 
shop  temperature  of  seventy  degrees  Fahrenheit.  Even  slight 
variations  of  temperature  will  cause  errors  of  importance  in 
the  length  of  an  ordinary  base-line,  the  change  in  length  per 
degree  of  temperature  and  per  unit  of  length  being  ahout 
0.0000066.  For  a  base-line  of  one  thousand  feet  and  a  varia- 
tion of  one  degree  tiie  change  in  length  would  be  eight  oue- 
bundredths  of  an  incii.  This,  it  is  true,  is  no  great  amount, 
but  there  is  always  a  liability  of  tliere  being  a  difference  of  as 
much  as  ten  degrees  between  the  average  temperatures  for 
measurements  made  on  two  different  days,  and  as  much  as 
two  or  three  degrees  In  a  single  measurement  of  a  base-line. 

In  using  a  steel  tape  it  is  better  to  start  from  the  one-foot 
mark  rather  lli  from  the  end,  imless  the  ring  be  placed  back 
of  the  zero-point. 

The  author's  method  of  measuring  a  base-line  on  compara- 
tively level  ground  is  to  run  in  a  line  of  stakes  of  at  least  three 
inches  by  one  inch  section  and  from  two  feet  upward  in 
length,  spaced  at  intervals  of  alxjut  ten  feet  and  put  in  to 
exact  line  and  level,  with  a  large  flat  headed  tack  driven  to 
line  on  each  stake,  and  the-  true  base-line  scratched  with  a 
knife  along  tlie  toj)  of  each  tack.  The  line  is  measured  by 
stretching  the  tape  with  a  uniform  pull  of  six  pounds  over  the 
line  of  stakes,  keeping  the  one- foot  mark  or  the  zero- mark,  as 
the  case  may  be,  over  the  centre  that  is  cut  on  the  hub  at  the 


330 


DE   PONTIBUS. 


end  of  till!  biuse-lliic,  iinil  scratching,'  with  ii  knife  on  the  tack 
where  the  fifty- foot  mark  on  the;  tape  comes,  IIum*  starting 
from  this  point  to  measure  anollier  forty-nine  or  fifty  feet, 
ami  so  on  until  the  centre  of  tiic  hulj  at  the  far  end  of  the 
Ixise-llne  is  reached.  The  next  time  that  the  line  is  measured 
tlje  lirst  length  should  be  thirty-nine  or  forty  feet,  so  »»  to 
avoid  using  the  same  tacks;  and  each  succeeding  first  length 
should  be  ten  feet  shorter.  This  not  only  involves  the  use  of 
fresh  tacks  for  each  measurement,  but  also  prevents  any 
manipulation  of  the  tape  so  as  to  make  the  partial  measure- 
ments agree  with  '       e  made  previously. 

In  case  that  a  Uly  level  line  cannot  be  obtaine<l,  the 

line  should  be  diviued  into  level  stretches,  and  where  each 
break  occurs  the  length  should  be  measured  on  the  incline 
and  corrected  afterwards  for  thi.'  effect  of  the  rise  or  fall  so  as 
to  obtain  tlie  true  horizontal  distiince. 

For  further  directions  as  to  measuring  b.iseliiics  with  a 
steel  tape,  the  re.'ider  is  referred  to  Johnson's  Surveying. 

The  ends  of  base-lines,  as  well  as  all  intermediate  points 
from  which  triangulation  operations  may  be  conducted,  should 
be  marked  by  solid  and  secure  hubs.  In  i)r()tected  places 
these  may  consist  of  six -inch  by  six-inch  timber,  three  feo»  or 
more  in  length,  driven  in  the  ground  and  ctit  otf»about  an  inch 
above  the  surface,  the  centre  being  marked  with  a  tack, 
across  which  are  cut  two  intersecting  lines  at  right  angles  to 
each  other. 

If  the  ground  be  .subjected  to  hard  freezing,  the  timber  should 
be  increased  in  section  to  eight  inches  by  eight  inches,  and  the 
length  should  be  such  that  it  will  penetrate  the  ground,  if 
possible,  about  three  feet  below  frost.  The  earth  around  the 
liub  location  should  be  excavat<'d  to  the  greatest  depth  of 
frost,  then  the  timber  should  be  driven  in  or  sunk  like  a  post 
and  well  tamped,  after  which  a  stout  timber  box  with  an  open 
bottom  and  a  strong  cover  should  be  placed  around  the  hub, 
and  the  earth  should  be  packed  around  the  outside  thereof. 
Finally,  the  box  should  be  filled  nearly  to  the  top  of  the  hub 
with  sawdust  or  dry  sand. 

In  case  that  the  ground  be  very  hard,  or  if  the  bed-rock  be 


1 


TIIIANOULATION. 


321 


near  the  surface,  it  will  bu  bent  to  surround  the  liub  with  con 
Crete,  and  protect  it  with  a  8ul)staiitiul  cover  of  some  kind  to 
prevent  displacement. 

If  driving  or  carting  is  to  be  carried  on  in  thevicinity  of  the 
hub,  the  latter  should  be  fenced  in  by  four  stout  posts  sunk 
into  the  ground  on  the  cornerH  of  a  s(|uare  of  seven  or  eight 
feet  on  a  side,  the  posts  projecting  high  enough  above  the 
ground  to  strike  a  wagon-box. 

In  locating  all  triangulation-hubs  it  is  essential  to  place 
them  so  that  the  operations  of  construction  will  not  obstruct 
th<'  view  of  the  transitman. 

If  there  is  a  possibility  that  any  of  the  hubs  will  be  disturbed 
by  the  ojierations  of  construction  or  in  any  ollie.  .iianiier,  sucli 
hubs  should  be  carefully  "tied  in"  by  reference  points  lo- 
cated souke  distance  away.  This  should  be  done  as  soon  as 
the  base-line  is  measured. 

Ther"  should  be  two  base-lines,  one  on  eiich  side  of  the 
river  and  both  on  the  same  side  of  the  bridge,  or  both  should 
be  on  the  same  side  of  the  river  with  one  above  and  the  other 
below  the  bridge.  Usually  it  will  be  found  satisfactory  to  lo- 
cate all  piers  from  one  point  on  each  base-line,  and  for  that 
reason  the  ends  of  the  base-line  should  l)e  ciiosen  so  that,  if 
possilde,  all  the  piers  can  be  seen  therefrom.  If  this  be  ira- 
jiracticable,  or  if  some  of  the  defleelions  would  for  any  rea- 
son be  too  small,  it  will  be  necessary  >o  put  in  and  use  in- 
termediate hubs  on  the  base-lines 

Base-lines,  whenever  it  is  practicable,  should  be  run  approx- 
imately at  right  angles  to  the  longitudinal  axis  of  the  bridge; 
but  this  is  by  no  means  essential,  and  it  is  folly  to  try  to 
iMake  the  intersection  exactly  at  right  angles,  except  in  the 
following  case,  which  represents  an  ideal  system  of  triaugu- 
lation  that  can  rarely  be  utilized,  on  account  of  the  existing 
conditions  of  shores,  and  obstructions  both  natural  and  arti- 
ficial. 

The  said  ideal  system  consists  in  running  four  base  lines, 
as  shown  in  Pig.  8,  all  exactly  al  right  angles  to  the  centre 
line  of  the  bridge,  and  laying  off  thereon  distances  equal  to 
those  {ipux  the  buse-Une   to  pier  'jenlres,  so  that  all  lines  pf 


DE    PONTIBUS. 


sight  will  intersect  Ihc  ceutre  line  at  angles  of  exactly  forty- 
five  degrees. 

The  a'lvantagc  of  this  sj'stem  lies  in  the  fact  that  all  the 
piers  are  located  by  direct  sight  without  having  to  measure  the 
angle,  the  only  angles  requiring  measurement  being  the  fotir 
right  angles  between  the  base-lines  and  the  centre  line  of 
bridge,  and  the  four  other  iingles  required  for  determining 
and  checking  the  distance  between  base-lines  along  the  bridge 
tangent. 

The  lengths  of  base-lines  for  ordinary  systems  of  tiiaugu- 
lalion  will  generally  be  regulated  by  local  conditions.     They 


4        3 


\  ^y«^  ' 


1   jo 


90 


\ 
\ 


\ 


\ 


*(1 


N  X          Pier 

/  _X^             \ 

y  y        ^v  Pidr 

/  /             \     / 


2         \^         ' 


/ 


\ 


\ 


\ 


\ 


Fig.  8. 


slioidd  usually  be  about  as  long  as  the  total  length  of  bridge, 
or,  when  there  is  a  base-line  on  each  side  of  the  river,  us 
long  as  the  perpendicular  distance  between  opposite  base- 
linos;  but,  if  necessary,  they  may  bo  made  as  short  as  seven 
tenths  of  same.  Too  short  base-lines  will  give  too  sharp  in- 
tersections, and  therefore  sometimes  loo  great  variations 
from  correctness  ;  nevertheless,  sharp  inlersections  can  be 
employed  at  times  by  taking  e.xiru  puins  with  the  work  and 
by  employing  an  extra  intersection  us  u  cheek,  in  case  that 
uuy  discrepancy  occurs. 


)i\ 


TRIANOULATION. 


323 


Aflor  llie  base-lines  are  measured  and  the  hubs  are  put  in, 
tlie  next  step  to  talce  is  to  measure  the  six  principal  angles  of 
thu  triangiilation.  Tliese  should  be  measured  with  I  he  great- 
est accuracy  continuously  around  the  limb  of  the  transit, 
making  fioin  ten  to  thirty  readings  of  each  angle,  according 
to  the  degree  of  refinement  required.  The  instrument  should 
be  graduated  for  accurate  work  as  fine  as  twenty  seconds,  or 
preferably  ten  seconds.  A  heavy  transit  with  a  good,  solid 
tripod  will  usually  give  better  results  than  those  obtained  by 
using  a  ligliter  instrument.  Tlie  sun  should  never  be  per- 
nutled  to  shine  on  !he  instrument  when  the  angles  are  being 
observed,  as  it  is  impossible  to  make  accurate  measurements 
under  such  a  condition. 

In  keeping  notes  of  triangulation-work  a  record  should  be 
made  of  the  date,  the  temperature,  the  condition  of  the  weather, 
the  direction  and  approximate  velocity  of  the  wind,  and  the 
names  of  the  trausitmaa  and  picketman. 

If  long  sights  are  to  be  taken,  the  picketman  .should  be  pro- 
vided with  a  pair  of  field-glasses  to  enable  him  to  see  the 
transitman's  signals  ;  otherwise  much  time  and  labor  may  1)0 
spent  to  no  purpose.  Long  sigiits  .diould  never  be  taken 
lowan^s  the  sua  when  it  can  be  avoided. 

Tiie  error  of  all  three  angles  in  each  cf  the  two  main  tri- 
angles should  not  exceed  two  seconds  in  important  work.  Of 
course  it  is  not  necessary  t;>  go  to  any  such  refinement  in 
.shoi  t-span  bridges  ;  but  in  very  long  ones  the  error  miglit  wcll 
In  reduced  as  low  as  one  second.  If  the  error  in  a  triangle  be 
I'oiuid  too  large,  it  maybe  possible  to  avoid  measuring  all  three 
angles  again  by  looking  over  the  notes  and  ascertaining  from 
tlie  weather  conditions  which  angle  is  most  likely  to  be  at 
fault,  then  mea.suring  this  angle  anew.  If  the  second  average 
angle  reduces  the  total  error  in  the  triangle  to  within  a  proper 
limit,  all  right ;  but  if  not,  the  other  t>vo  angles  will  also  have 
to  be  measured  a  second  time. 

On  the  same  principle,  if,  in  a  group  of  measurements  of 
one  angle,  one  or  two  readings  be  fouud  to  differ  greatly 
from  the  others,  they  nuiy  be  throwu  out  when  obtaining  the 


324 


DE   PONTIBUS. 


It  sometimes  happens  tliat  both  intersections  of  the  bridge 
tungent  with  the  base-lines  cauuot  be  seen  from  one  end  of  one 
of  the  hitter.  In  this  case  it  will  be  necessary  to  put  in  a  hub 
on  the  bridge  tangent  far  enough  ahead  of  the  hidden  point  to 
clear  the  obstruction,  triangulate  to  it,  and  measure  the  exact 
distance  from  it  to  the  hub  on  the  base-line.  This  expedient 
was  necessary  in  the  triangulation  for  the  author's  Jefferson 
City  highway  bridge. 

A  check  on  the  accuracy  of  the  triangulation  work  is  ob- 
tained by  comparing  the  two  computed  lengths  of  the  bridge 
tangent  between  the  intersections  thereof  with  the  base  Hues, 
or  between  one  such  intersection  aud  a  fixed  point  on  tlie 
tangent  on  the  other  side  of  the  river.  The  disagreement  in 
these  two  measurements  should  be  withiu  the  lin.il  of  one  half 
of  au  inch  to  the  one  thousand  feet.  To  show  how  accurately 
such  work  can  be  done,  the  author  would  state  that  for  the 
Jefferson  City  bridge  he  gave  his  resident  engineer  instruc- 
tions to  allow  no  variation  from  correctness  exceeding  three 
eighths  of  au  inch  iu  either  the  mdin  triangulation  itself  or  in 
the  intersections  for  pier  centres.  His  instructions  were  fol- 
lowed so  faithfully  that  no  error  exceetling  three  sixteenths  of 
an  inch  was  allowed  to  pass  in  any  part  of  the  work.  The 
•whole  field-force  once  lost  au  entire  haif-day  iu  rectifying  an 
error  of  one  half  of  an  inch  iu  the  intersections  for  a  i)ier 
centre.  This  is  an  excellent  record  ft)r  accuracy,  considering 
that  the  dista.  ce  between  base  lines  on  the  bridge  tangent  was 
a  little  over  fi..een  hundred  feet.  The  author  is  generally  not 
so  rigid  iu  his  retiuirements  for  exactness  as  he  was  in  this 
case,  the  reason  for  such  strict  instructions  being  the  fact  that 
this  was  the  resident  engineer's  first  experience  in  important 
triangulation. 

The  triangulation  for  the  author's  Sioux  City  bridge,  made 
by  LeeTreadwell,  Mem.  Am.  Soc.  C.  E.,  with  a  bridge  tangent 
about  twenty-two  hundred  feet  long  between  base-lines,  was 
probably  just  as  accurate  as  that  for  the  Jefferson  City  bridge, 
because  the  errors  in  distances  between  pier-centres  measured 
pn  top  of  the  falsework  n-  re  actually  inappreciable. 


TRIANGULATION, 


326 


After  the  main  triungulation  for  a  bridge  is  finished,  the 
next  step  is  to  compute  tiie  angles  to  the  various  points  on  the 
piers  that  will  be  needed  during  llie  sinking.  For  a  single 
cylinder  pier  it  will  suffice  to  triangulate  to  the  centre  only, 
and  for  a  pier  composed  of  two  cylinders  a  triangulation  to 
the  centre  of  each  cylinder  will  be  enough  ;  but  for  a  rectan- 
gular pier  it  will  be  necessary  to  locate  not  only  the  centre, 
but  also  anotlier  point  near  the  periphery,  in  order  to  prevent 
tlie  pier  from  being  rotated  ab(;ut  its  vertical  axis  in  going 
down.  After  the  calculations  are  completed  a  triangulation- 
sheet  should  be  prepared,  on  which  should  be  shown  all  of 
the  triangulation  with  the  various  distances  on  all  lines  and 
the  exact  angles  for  all  deflections. 

Foresiglits  should  next  be  located  for  the  bridge  tangeni 
and  for  all  pier  points,  so  that  the  Iraiisitman  fliall  never  be 
under  the  necessity  of  turning  oil  an  angle  when  locating  a 
pier.  The  position  for  any  foresight  is  generally  determined 
by  convenience,  but  it  should  be  chosen  so  as  to  avoid  any 
IMobabilify  of  disturbance.  I-'ach  foresight,  which  consists  'f 
a  substantial  wooden  targt  L,  'is  located  by  turning  off  the 
propei  angle  from  the  l)ase-line,  and  is  then  fixed  immovably 
in  position,  iifier  whic  i.  a  series  of  from  ten  to  tiiirty  readings 
of  the  angle  is  made,  thi  (jornspouding  et'iitie  lines  being 
marked  on  the  target.  Tlie  average  of  all  of  these  centre 
lines  is  then  detenu ined,  and  is  assinued  to  be  the  true  centre, 
whicli  is  marked  ( onspicuously  on  the  target.  Each  target  is 
to  be  marked  also  with  its  characteristic  letter  or  n\imber,  so 
that  its  individuality  ni?iy  be  recognized  by  the  transilman 
from  the  most  dist  I'  ■  oint  of  observation.  The  angles  for 
determining  the  conoei  centre  of  any  target  sliould  be  laid  off 
continuously  on  the  limb  of  the  transit.  All  foresights  should 
be  inspected  occasionally  so  as  to  see  that  they  have  not  beer 
disturbed,  although  any  disturbance  will  be  discovered,  the 
firat  time  that  the  foresight  is  used,  by  the  three  lines  failing 
to  intersect  in  a  point. 

When  piers  are  to  be  built  in  open  coffer-dams,  'he  work  of 
locating  them  is  comparatively  simple ,  for  when   'hey  are 


32G 


1)E   PONTIBUS. 


once  located  little  or  no  movement  takes  place  afterwards. 
But  when  piers  are  to  be  sunk  by  the  pneumatic  process  or  by 
open  dredging,  great  care  must  be  taken  at  every  step,  because 
the  pier  is  always  eitiier  moving  or  liable  to  move  at  any  mo- 
ment. In  sinking  piers  by  either  of  the  two  last-mentioned 
processes,  the  resident  engineer  should  keep  such  notes  that 
from  them  he  can  report  daily  as  to  the  exact  horizontal  posi- 
tion of  the  cutting  edge  of  the  caisson,  the  position  of  the  top 
of  the  pier,  the  elevation  of  the  cutting  edge,  the  iucliuatiou 
of  the  axis  of  the  pier  to  the  vertical,  and  the  amount,  if  any, 
that  tlie  pier  has  been  revolved  around  its  vertical  axis.  Tlie 
Contractor  can  contluct  his  operations  with  much  more  cer- 
tainty of  landing  the  pier  in  its  true  position,  if  he  be  kept 
informed  as  to  its  relative  position  every  day. 

If  temporary  staging  be  used  around  the  jner,  from  wliich  to 
conduct  the  operations  of  construction,  keeping  track  of  the 
various  motions  of  the  pier  will  be  a  comparatively  easy  task, 
for  the  approximate  alignment  can  be  obtained  from  tempo- 
rary points  located  on  the  staging,  which  points,  however, 
need  occasional  checking  to  see  that  the  staging  has  not  shifted 
sliglitly. 

If  there  be  no  st'iging,  all  locations  will  have  to  be  made  by 
triiingulation,  and,  as  before  staled,  two  points  on  each  pier 
will  be  needed  in  order  to  detect  rotation.  When  the  caisson 
has  reached  a  considerable  depth,  however,  the  liability  to 
rotate  is  greatly  lessened. 

After  all  that  may  be  said,  the  work  of  keeping  the  pier  in 
correct  position  will  be  dependent  on  local  conditions  and  many 
varying  recpiirements. 

In  respect  to  the  levels,  care  should  always  be  taken  to  pre- 
serve su('h  measurements  as  will  enal>le  the  leveller  to  keep  a 
record  of  the  vertical  distance  fvom  the  cutting  edge  to  the 
top  of  the  crib  at  each  of  the  four  corners.  This  will  be 
necessary  in  order  to  determine  how  much  the  said  cutting 
edge  is  out  of  level. 

In  giving  the  final  elevations  for  the  copings  of  the  piers, 
it  will  sometimes  be  found  necessary  to  lake  very  lonij  fore- 


TRlANGlTLATION. 


33 


Oi^ 


sights,  owiug  to  the  impracticability  of  setting  up  the  level 
near  the  piers.  In  such  cases  a  backsight  should  be  taken  to 
a  bench-mark  about  the  same  distance  from  the  iustvumeut  as 
the  pier  is  therefrom,  and  in  the  opposite  direction,  so  as  to  off- 
set  a  possil)le  slight  lack  of  adjustment  in  the  level,  and  to 
compensate  for  the  curvature  of  the  earth. 


CHAPTER  XXIV. 


OFFICE -PRACTICE. 


As  there  has  been  almost  nothing  yet  written  concerniii!?  the 
way  in  which  work  is  handled  in  a  Consulting  Engineer  s  of- 
fice, the  author  has  concluded  to  close  this  little  treatise  with 
a  chapter  on  "  Oftice  Practice  "  ;  and  as  no  two  engineers  pur- 
sue exactly  the  same  methods,  and  as  the  author  is  naturally 
more  familiar  with  his  own  than  with  those  of  others,  he  will 
deal  herein  solely  with  the  established  practice  of  his  own  of- 
fice, which  practice  is  the  outcome  of  over  ten  years  of  special 
effort  to  secure  the  best  possible  results  both  expeditiously  and 
economically. 

LAYING  OUT   WORK. 

This  chapter  being  confined  entirely  to  oftlce-work,  it  will 
be  assumed  at  the  outset  that  all  such  field  data  as  profiles, 
maps,  plats  of  borings,  etc.,  have  been  secured. 

In  bridge-work  it  is  necessary  to  determine  the  following 

First.  The  Purpose  for  which  the  Structure  is  to  be  used.— This 
being  settled,  there  ensues  the  fixing  of  the  live  load,  the 
clearance  between  trusses,  and  the  clear  heiglit  above  base  of 
rail  or  surface  of  roadway. 

Second.  The  Clear  Height  between  Standard  High  Water  and 
the  Lowest  Part  of  Structure.  -  If  the  stream  be  a  navigable  one, 
the  minimum  clearance  will  be  regulated  by  the  requirements 
of  the  War  Department.  In  other  cases  the  clear  h(!ight  will 
depend  on  the  required  elevation  of  grade  of  railroad  or  road- 
way, provided  that  the  lowest  part  of  the  superstructure  will 
never  offer  any  ol)struclion  to  tioatiiig  drift  or  ice  during  the 
highest  floods.  The  minimum  clearance  should  preferably 
be  ten  feet,  and  never  less  than  five. 

328 


OPPICE-PRACTICE. 


329 


Where  a  low  bridge  is  required  over  a  navigable  stream, 
some  oue  of  the  various  kinds  of  laov.ible  bridges  described 
iu  Chapters  IX  aud  X  must  be  used  ;  but  for  all  ordinary 
cases  the  rotating  draw  is  the  most  suitable  type. 

Tliird.  Best  Span  Lengths  to  adopt. — In  many  cases  there  will 
be  no  choice  as  to  span  lengths,  whicli  are  liable  to  be  deter- 
mined by  such  conditions  as  the  reqidremcnts  of  the  War  De- 
partment, obstraction  of  stream  by  piers,  danger  fronj  wasli- 
out  during  erection,  etc.;  but,  wiicre  the  designer  has  any 
choice  in  the  mailer,  he  siiould  be  governed  by  the  principles 
of  economy  laid  down  in  Chapter  Ilf,  taking  care,  however, 
lliiit  he  does  not  violate  any  of  the  principles  of  esthetics  given 
in  Chapter  IV,  unless  he  be  forced  to  do  so  by  circumstances 
that  are  absolutely  beyond  liis  control.  As  stated  in  Chapter 
III,  the  greatest  possible  economy  will  exist  when  the  cost  of 
each  pier  is  equal  to  one  half  of  the  cost  of  tlie  trusses  and 
lateral  systems  of  the  two  spans  which  it  hel[)3  to  support. 
The  determination  of  these  economic  conditious  is,  of  course, 
a  matter  of  cut  and  try;  but  after  a  few  trials  tlie  economic 
span  length  can  be  approximated  very  closely.  In  making 
such  calculations  the  trial  weights  of  trusses  and  laterals  can 
be  found  with  sufficient  accuracy  by  taking  a  span  of  known 
weight  and  computing  therefrom  tlie  weights  for  the  spans  of 
the  trial  lengths  by  the  following  methods  : 

A.  The  weight  per  foot  of  the  lateral  system  is  directly  pro- 
portional to  the  span  length,  provided  that  the  superstructure 
is  not  changed  in  width,  which  is  generally  the  case.  Should 
the  width  be  changed,  the  new  weight  will  have  to  be  modi- 
fied accordingly,  under  the  assumption  that  the  weight  varies 
about  half  as  rapidly  as  does  the  width. 

B.  To  find  the  truss  weight  W  i>er  lineal  foot  of  span  of 
length  I'  from  the  corresponding  known  weight  W  of  span  I, 
the  following  approximate  but  quite  accurate  empirical  for- 
mula may  be  used : 

This  will  give  approximately  the  weight  per  foot  of  trusses 


330 


l)K    PONTIIJUS. 


for  any  spun  length,  provided  the  live  load  per  lineal  foot  re- 
main uncluinged, 

C.  Tofliid  for  liny  span  length  the  truss  weight  7"  per  lineal 
fool  for  a  total  load  p'  per  lineal  foot  from  the  corresponding 
known  weight  7' for  a  load  /?,  the  following  approximate  em- 
pirical formula  may  be  used  . 


•=.-('+^^)- 


This  is  quite  accurate  for  all  ordinary  spans,  but  for  very 
long  ones  it  gives  too  great  a  variation  between  7"  and  T. 

After  finding  the  value  of  7",  the  value  used  forp'  should 
be  checked;  and  if  there  be  any  serious  disagreement  between 
the  value  assumed  and  that  found,  the  substitution  in  the 
formula  should  be  made  anew,  and  so  on  until  a  satisfactory 
agreement  between  the  said  values  of  jt'  be  obtained. 

Fourth.  General  Layout  of  Structure.  —The  general  layout 
should  consist  of  a  profile,  a  plan,  and  enough  cross-sections 
to  illustrate  properly  the  entire  substructure,  superstructure, 
and  approaches,  all  being  made  to  exact  scale.  For  long 
crossings,  a  scale  of  one  fortieth  of  an  inch  to  the  foot  is  the 
most  satisfactory,  but  for  short  crossings  the  scale  should  be 
made  larger. 

The  proportioning  of  the  skeletons  of  the  trusses  should  be 
done  in  accordance  with  the  suggestions  given  in  several  of 
the  preceding  chapters,  and  the  dimension 3  of  the  piers  should 
be  determined  by  the  principles  established  in  Chapter  XXII. 

Each  general  layout  should  give  the  following  information  : 

Elevations  of  bed-rock,  low  water,  standard  high  water, 
extreme  high  water,  lowest  part  of  structure,  grade-lines  and 
tops  of  piers,  lengths  of  all  spans  between  centres  of  end-pins 
or  centres  of  bearings,  distances  between  centres  of  piers,  all 
leading  dimensions  of  piers,  heights  of  trusses,  and  lengths 
and  kinds  of  approaches. 

As  soon  as  the  general  layout  is  completed  and  finally 
adopted,  the  computations  of  stresses  and  sizes  of  members  of 
spans  may  be  begun. 


OPFICE-l'RACTICK. 


331 


For  elevated  railroads  it  is  necessary  to  delcrmiae  the 
following  ; 

First.  The  number  of  tracks  on  the  various  portions  of  the 
line,  and  tlie  clearances  over  streets  and  alleys. 

Second.  The  live  load  per  track  to  be  carried  by  the  struc- 
ture. 

2%ird.  The  location  of  the  line,  whether  in  the  streets  or  on 
private  property. 

Fourth.  The  style  or  styles  of  girder  construction.  In 
some  locations  the  City  Ordinances  may  require  open-webbed 
girders,  :is  these  shut  out  less  light  than  do  solid  plate  girders, 
while  in  otlier  locations  the  plate  girders  would  be  per- 
missible. 

Fifth.  The  location  of  columns,  whether  in  the  street  or  on 
the  curbs,  also,  for  location  on  private  property,  the  number 
of  columns  per  bent. 

Sixth.  The  economic  span  length.  As  indicated  in  Chapter 
III,  the  greatest  economy  will  exist  when  the  cost  of  the 
longitudinal  girders  is  equal  to  the  cost  of  the  cross-ginlers, 
columns,  and  pedestals.  Where  the  columns  are  located  in 
the  street  or  on  the  curbs,  due  consideration  must  be  given  to 
the  probable  cost  of  removing  underground  obstructions,  such 
as  water-pipes,  gas-mains,  etc. 

With  these  points  all  settled,  the  calculations  for  propor- 
tioning all  parts  of  the  structure  may  be  proceeded  with. 

Where  the  structure  is  on  a  curve,  it  is  best  to  make  the 
bents  radial  whenever  practicable.  The  exact  location  of 
each  coluuni  should  be  figured  from  certain  known  lines,  and 
all  ordinates  for  same  should  bo  indicated  on  the  layout. 
Much  careful  study  should  be  given  to  the  work  of  establish- 
ing each  feature  of  the  layout ;  for,  if  mistakes  be  made 
therein,  they  are  liable  to  cause  great  delay  and  expense 
later  on. 

Roof-trusses  and  steel  buildings  will  not  be  treated  in 
this  book,  as  it  deals  mainly  witJi  bridges,  viaducts,  and 
elevated  railroads.  The  office  work  connected  with  the  de- 
signing of  roofs  and  steel  buildings  will,  however,  not  differ 


332 


»E    PONTIBUS. 


csscutially  from  tbat  pertuiuing  to  the  desiguiog  of  the  other 
structures. 


CALCULATIONS. 

After  the  leading  features  of  any  proposed  structure  have 
been  determined,  and  after  tlje  general  layout  thereof  is  com- 
pleted, the  next  step  to  itikc  is  the  making  of  the  calculations 
ncoessiiry  to  dcternuuc  the  stresses  in  all  the  parts  and  the 
proper  sizes  for  same. 

For  convenience  in  making  to  correct  scale  pen-sketches 
of  the  various  portiims  of  the  design,  the  author  uses  a  cross- 
secliou  paper  divided  into  one-qiiaitor-inch  squares,  the  slieets 
l)eii)g  ten  and  a  half  inches  wide  by  sixteen  inches  long,  which 
size  experience  has  shown  to  be  the  most  satisfactory.  At 
tlie  head  of  each  page  are  written  the  date,  title  of  structure, 
and  name  of  computer. 

At  the  beginning  of  each  set  of  calculations  the  following 
general  data  for  spans  are  given  : 

First.  Lengtli  of  span. 

Second.  Number  of  panels. 

Third.  The  various  truss  depths. 

Fourth.  Perpendicular  distance  between  central  planes  of 
trusses. 

Fifth.  Live  load  or  loads  to  be  used. 

Sixth.  Wind  loads  for  both  upper  and  lower  lateral  systems. 

Seventh.  Spacing  of  stringers. 

The  dead  loud  from  the  track  and  ties  in  railroad  bridges  or 
from  the  timber  floor  or  pavement  in  highway  bridges  is  first 
determined,  using  the  unit  weights  of  materials  given  iu 
Chapter  XIV;  then  the  stringers  or  longitudinal  t'rders  are 
figured  and  proportioned,  after  which  their  weights  and  that 
of  their  bracing  are  computed. 

Next  the  floor-beams  or  cross-girders  are  proportioned,  and 
their  weights  are  figured.  From  all  these  weights  the  weight 
per  lineal  foot  of  the  metal  in  the  floor  system  is  next  found. 

As  the  lateral  system  can  nearly  always  be  designed  before 
the  trusses,  it  is  generally  best  to  compute  the  weight  per 
Jiueal  foot  of  the  entire  lateral  system  before  the  trusses  are 


OPFICK-PRAOTICE. 


3;j:j 


touched,  because  the  dciid  load  fur  llie  hitler  will  be  affected 
by  the  weight  of  the  former. 

Next  it  is  necessary  to  assume  the  weij^ht  of  metal  per  lineal 
foot  for  tlie  trusses,  using,  if  necessary,  the  formula'  given 
previously  in  this  chapter.  This  completes  the  data  for  the 
preliminary  dead  load,  which  will  consist  of  llie  following 
items  : 

First.  Flooring  (timber,  traclt,  pavement,  etc.). 

Second.  Floor  system  (stringers,  stringer-bracing,  and  Hoor- 
beams). 

Third.  Lateral  system  (upper  and  lower  lateral  systems, 
vertical  sway-bracing,  and  portal-bracing). 

Fourth.  Trusses. 

In  making  up  the  dead  load,  the  end  tloor-l)cnms  and  pedes- 
tals must  not  be  included,  as  their  weight  produces  no  bending 
moment  on  the  span. 

The  dead-load  stres.ses  in  trusses  are  always  found  analyti- 
cally for  spans  with  parallel  chords  and  equal  panel  lengtiis; 
but  for  all  other  cases  they  are  determined  graphicjilly,  and 
are  checked  by  a  single  numerical  calculation  at  the  member 
where  the  graphics  stop. 

Whenever  it  is  practicable,  in  making  arithmetical  com- 
putations, the  slide-rule  is  employed.  For  ordinary  work,  in 
which  tlie  totid  stresses  can  be  written  with  six  figures,  a 
twelve-inch  slide-rule  will  give  the  stresses  accurately  in 
thousands  of  poumls  ;  but  where  the  stresses  are  greater, 
Thacher's  cylindrical  slide-rule  is  employed. 

The  live-loud  stresses  are  found  by  the  method  explained  in 
Chapter  XIX. 

The  computation  of  all  stresses  found  analytically  is  facili- 
tated by  determining  the  trigonometrical  functions  involved 
in  the  calculations,  and  multiplying  the  panel  loads  by  them. 
By  setting  these  products  on  the  slide-rule  and  using  the 
proper  tabulated  coefiicients,  it  is  often  practicable  to  read  off 
a  large  series  of  stresses  without  resetting  the  slide. 

The  dead-load  stresses  and  the  live-load  stresses  are  written 
on  separate  diagrams  on  the  calculation-sheets. 

Xhp  impact  stresses  arp  f opnd  from  the  Uve-load  stresses  l)y 


3;u 


J)K   PONT  I  BUB. 


slide-rule  from  the  fornmlie  given  in  eitlicr  Chapter  XIV  or 
Clmpter  XVI.  us  the  (luse  nmy  be,  or  from  llie  corresponding 
tublcH  at  the  end  of  the  book,  and  are  written  on  a  sepanile 
diagram. 

Next  are  computed  all  tic  wind-stresses  which  could  possi- 
bly  allcct  I  he  sizes  of  llu;  sections  of  inain-tniss  members,  and 
these  are  recorded  eitiicr  on  a  separate  diagram  or  on  one  of 
those  already  preiwired,  in  the  latter  case  care  being  talien  to 
indicate  that  eacli  such  stress  is  marked  as  a  wind-load  stress. 

Next  the  various  combinations  of  all  stresses  are  made  and 
recorded  on  a  new  diagram,  after  which  the  reqinred  sections 
of  nil  main  meml)ers  are  figured  according  to  the  specitica- 
tioua,  and  are  recorded  on  the  .same  diagram;  then  the  actual 
sections  are  proportioned  and  recorded  there  also, 

The  exact  lengths  of  all  members,  including  camber  a"ow. 
anoes,  are  next  figured  and  recorded  on  the  lasl-menlioned 
diagram. 

Next  the  weight  of  metal  in  the  trusses  is  estimated.  For 
preliminary  estimal'is  'he  weights  of  details  are  percenlaged 
from  recorded  results  of  previous  similar  estimates  ;  but  if 
the  structure  be  of  an  unusual  type  or  size,  the  details  are 
sketched  and  tlieir  weiglits  are  computed. 

Next  the  total  weight  of  metal  in  the  structure  is  figured, 
and  tlie  dead  load  is  checked.  If  it  does  not  agree  with  tliat 
a.ssumed  witlii'j  the  limit  of  error  set  in  the  si)ecifications,  a 
new  dead  load  is  assumed,  and  the  entire  computations  of 
total  stresses,  sections,  and  truss  weights  are  made  anew.  It 
is  very  seldom,  liowever,  that  it  is  necessary  to  make  thes(! 
calculations  more  than  once,  owing  to  the  great  mass  of 
accumulated  data  concerning  weights  of  metal  in  all  kinds  of 
bridges. 

lu  making  any  set  of  calculations  the  computer  .should 
check  back  on  his  work  at  short  intervals,  so  as  to  see  that  no 
crior  lias  been  made,  because  the  effects  of  such  errors  often 
extend  over  all  succeeding  computations. 

In  determining  .stresses  graphically,  the  frame-diagram  shoidd 
be  laid  out  on  as  large  a  scale  as  is  convenient,  and  the  load- 
diagram  should  be  made  a^  aroall  as  practicable  ;  for  the  lar^e 


OFFICK-PRACTICE. 


335 


(id, 

I  at 

a 

of 

It 

est; 

of 

of 


frame  pjlves  great  acciinicy  in  Inclinutions  of  members,  which 
in  llie  all-iinportant  point  in  graphicul  computations,  and  the 
small  load-diagrimi  conilnes  the  graphics  ton  reasonable  space. 
If  tlie  inclinations  are  correct,  nccurate  results  will  be  obtained 
with  a  very  small  loud-diagram.  Tlic  autlior's  limits  of  error 
for  graphical  work  arc  one  quarter  of  one  per  cent  at  mid- 
span  and  one  per  cent  at  the  far  end  of  span.  Siiould  the 
error  exceeci  these  limits,  the  graphical  work  has  to  be  done 
anew.  Smooth  paper,  sharp  pencils,  true  triangles,  and  per- 
fect straightedges  are  necessary  to  secure  good  results,  to 
which  list  should  be  added  painstaking  accuracy  in  every 
manipulation  of  the  appliances. 

All  calculations  on  the  standard  sheets  are  made  in  black 
copying-ink  ;  and  when  they  are  checked  l)y  another  computer, 
as  is  the  invariable  oustoivi  in  the  author's  oOice,  all  check- 
marks and  corrections  r.re  made  in  red  ink,  and  each  page 
checked  is  so  marked  and  initialed  by  the  checking  ccmpulei', 
who  not  only  verifies  all  the  numerical  calculations,  but  also 
follows  carefully  each  step  in  the  design  so  as  to  guard  against 
all  possible  errors.  Tlie  work  of  checking  is  greatly  facilitated , 
if  all  the  steps  taken  are  indicated  plainly',  so  that  they  can  be 
easily  followed  by  the  checker.  Each  result  checked  is  ticked 
off  with  red  ink. 

MAKINO   DRAWINGS, 

Owing  to  the  necessity  for  iiaving  several  copies  made  of 
each  drawing,  the  latter  is  lirst  laid  out  in  pencil  on  brown 
paper,  and  is  copied  in  ink  on  tracing-cloth.  In  some  simple 
designs,  however,  the  pencilling  is  done  directly  on  the  trac- 
ing-cloth ;  but  this  is  the  exception  rather  than  the  rule.  For 
convenience  in  handling  and  filing,  it  is  very  desirable  to  have 
all  drawings  made  of  a  uniform  size.  After  several  years  of 
experience,  a  size  of  twenty-nine  inches  in  width  and  thirty- 
eight  inches  in  length  has  been  adopted  as  best  suited  for 
bridge  pi  ins.  This  .size  may  be  used  for  all  detail  drawings 
and  stress- diagrams,  but  it  is  often  necessary  to  increase  the 
length  for  profiles  and  general  drawings.  The  drawing  is 
always  made  on  the  rough  side  of  the  tracing-cloth,  as  it  is 


336 


DE    PONTIBUS. 


often  convenient  to  do  a  couaideiabk  amount  of  drawing  and 
writing  in  pencil  on  tlie  sheet.  AnotUer  reason  for  using  liie 
rough  side  is  that  any  erasure  shows  iess  thereon  than  it  would 
on  the  smooth  side,  and  it  is  often  necessary  to  do  considernble 
erasing  on  tracings. 

As  before  stated,  the  first  drawings  to  be  made  are  the  gen- 
eral profile  and  plan  with  cross-sections,  to  establish  all  the 
main  dimensions  of  the  structure.  These  drawings  can  be 
prepared  before  the  computations  are  finished.  Next  come 
the  streSvS-diagrams,  which  should  contain  the  cambered  lengliis 
of  all  members,  the  dead  load,  liv<j  load,  impact  and  wind-load 
stresses,  and  the  greatest  combinations  of  same,  the;  sections 
requiied  a'ui  those  used  for  eacli  main  member,  and  the  fol- 
lowing general  data: 

First.  Length  of  span  from  centre  to  centre  of  end-pins. 

SeconJ.  Number  of  panels. 

Third.  Perpendicular  distance  between  central  planes  of 
trusses. 

Fourth.  Depths  of  trusses. 

Fifth.  Dead  load  for  floor  system  per  lineal  foot  of  span. 

Sixth.  Dead  load  fur  trusses  per  liuea.  foot  of  span. 

Seventh.  Live  load  for  stringers  per  lineal  foot  of  span. 

Eighth.  Live  load  for  floor-beams  per  lineal  foot  of  span. 

Ninth.  Live  load  for  trusses  per  lineal  foot  of  span. 

Tenth.  Wind  load  on  upper  lateral  system  per  lineal  foot  of 
span. 

Eleventh.  Wind  load  on  lower  lateral  system  pe>  lineal  foot 
of  span. 

Twelfth.  Clearance  required  above  base  of  rail  or  floor. 

TJiirteenth.  Kinds  of  materials  to  be  employed  in  all  parts 
of  structure. 

Fourteejith.  Diameters  of  rivets  to  be  used. 

The  stress- diagram  proper  may  be  simply  a  line-drawing, 
each  main  member  being  repreeentcd  by  u  single  right  line,  or 
all  the  main  menibers  may  be  drawn  to  scale  by  means  of  their 
periphery-lines.  The  latter  method  ia  generally  adopted  be- 
cause of  the  improved  appearance  of  the  sheet  which  il  affords. 
The  scale  for  any  stress-diagram  should  be  large  enough  to 


OFFICE-PRACTICE. 


337 


give  plenty  of  room  between  puuel  points  to  contain  all  the 
necessary  writing. 

After  the  stress-diagrams  are  completed,  the  detail  drawings 
arc  begun.  There  is  considerable  differouce  in  the  methods 
employed  by  consulting  engineers  to  convey  to  manufacturers 
an  understanding  of  the  design  whicli  they  desire  to  have 
executed  in  the  shops.  Some  insist  that  the  only  proper 
method  for  the  engineer  to  pursue,  if  he  desires  his  details  to 
be  followed,  is  to  make  complete  working  or  shop  drawings, 
ready  to  be  turned  over  to  the  template  makers,  while  others 
prefer  to  make  what  are  termed  general  detail  drawings,  whicli 
show  to  exact  .s(;ale  all  the  details,  and  give  all  in>|)<)itant 
dimensions  and  the  number  of  rivets  in  each  connection,  l)Ut 
wliich  do  not  locate  each  rivet  by  figures,  leaving  the  working 
druwings  to  i)e  made  l)y  the  manufacturer.  When  the  latter 
method  is  adopted  the  working  drawings  must  be  sent  in  du- 
plicate to  the  engineer  for  his  approval  before  any  of  the  work 
is  sent  into  the  shops,  the  said  drawings  being  checked  by  the 
engineer's  assistants,  not  only  to  see  that  they  agree  in  every 
important  particular  with  the  original  drawings,  l)ut  also  to 
make  sure  that  they  contain  no  errors  of  any  kind. 

The  latter  metliod  is  the  one  which  the  author  invariably 
employs,  and  for  adopting  it  he  gives  the  following  reasons  : 

First.  Each  bridge-shop  has  certain  methods  of  doing  work, 
which  demand  thai  the  working  drawings  be  made  in  accord- 
ance therewith  ;  otherwise  the  cost  of  the  manufacture  is 
materially  increased.  These  methods  cannot  be  considered 
by  the  engineer,  who  has  neither  the  time  nor  the  inclination 
to  go  to  the  (rouble  of  acquainting  himself  with  the  various 
methods  of  all  the  leading  bridge-shops  of  the  country. 

t>econd.  The  miture  of  the  work  of  a  consulting  engineer  is 
not  such  as  to  ju.stify  him  in  keeping  together  enough  trained 
draftsmen  to  execute  with  sutlicient  rapidity  the  large  amount 
of  drawing  necessary,  if  the  first-named  method  be  followed. 

TJiird.  The  capacity  for  accomplishing  work  in  a  consulting- 
engineer's  office  when  the  second  method  is  employed  is  prob- 
ably three  times  as  great  as  it  would  be  were  the  first  method 
adopted, 


338 


PE    PONTIBUS. 


Fourth.  With  llie  careful  aud  thorough  system  of  checking 
shop-drawings  in  vogue  iu  the  author's  office,  all  the  advan- 
tages to  be  gained  by  making  complete  working  drawings  are 
obtained  by  the  much  simpler  method  of  making  complete 
detail  drawings. 

Fifth.  The  manufacturer  always  appears  lobe  better  pleased 
and  satistied  if  the  making  of  the  sliopdrawings  be  left 
to  him ;  and  the  work  of  manufacturing  the  metal  proceeds 
more  smoothly  in  consequence. 

In  starting  a  detail  drawing,  the  first  thing  to  be  done  is  to 
lay  out  a  sheet  of  standard  size.  If  the  subject  be  a  framed 
structure,  such  as  a  bridge  or  roof  trns.s,  it  will  greatly  econo- 
mize space  on  the  drawing  if  the  skeleton  frame  be  laid  out  on 
u  small  scale,  say  thrce-ciglilhs  or  one-half  incli  to  the  foot, 
thus  giving  the  proper  inclinations  of  all  membei"8,  and  if  the 
details  at  all  the  panel  points  and  connections  be  made  to  a 
larger  scale,  say  three  (juarters  of  an  inch  or  an  inch  to  the 
foot.  The  centre  ofgnivily  lines  of  all  nudn  members  should 
coincide  with  the  lines  of  the  skelet<ni  diagram.  For  the 
details  of  ordinary  bridges  the  scales  just  mentioned  will  be 
found  the  most  satisfactory. 

It  is  a  very  common  error  among  bridge-draftsmen,  when 
two  different  scales  are  used,  to  make  the  principal  lines  of 
the  main  members  continuous  between  panel  points,  thus 
exaggerating  the  apparent  size  of  the  said  members.  This  is 
entirely  wrong,  and  is  often  the  source  of  serious  errors  in  the 
shops.  In  such  drawings,  the  main  members  should  be  broken 
oil  bciore  their  principal  lines  meet  midway  between  the  panel 
points ;  and  it  is  often  advisable  to  show  a  section  of  the 
member  between  the  broken  ends. 

After  deciding  upon  the  scales,  the  next  step  is  to  determine 
what  portions  of  the  structure  are  to  be  showi  on  each  .sheet, 
if  more  than  one  is  to  be  made,  and  what  is  the  best  i)o.ssible 
arrangement  for  all  details  on  each  sheet  so  as  to  fill  it  uni- 
formly and  allow  ample  space  for  illustrating  each  detail  in 
the  requisite  number  of  views.  For  short  spans,  up  to  say  two 
hundred  feet,  by  carefully  arninging  the  details,  everything 
can  besljown  clearly  on  a  standard  sheet  of  twenty-nine  inches 


wo 

of 

tak 

as 
be 
so 


OFFICli-PUACTIC'E. 


339 


by  thirty-uiglit  inches.  The  sizes  of  all  connecting-plates, 
stay-plates,  lacing-bars,  connecting-angles,  pins,  tillers,  rivets, 
etc.,  should  be  given,  also  those  of  all  main  members;  and 
the  exact  spacing  from  back  to  back  of  all  angles,  channels, 
and  webs,  forming  the  various  members,  should  be  clearly 
indicated.  The  packing  at  all  panel  points  sliould  be  shown, 
and  the  exact  spacings  therefor  should  be  given  by  figures. 

There  should  be  indicated  also  all  leading  dimensions,  such 
as  the  exact  cambered  lengths  from  centre  to  centre  of  pin- 
holes for  all  truss  members;  the  vertical  distance  from  centre 
of  bottom-chord  pins  to  base  of  rail;  the  vertical  distance  from 
centre  of  bottom-chord  pins  to  bottom  of  floor-beams;  the 
vertical  distance  from  base  of  rail  to  top  of  masonry;  the 
clearance  required  above  base  of  rail;  the  spacing  of  anchor- 
bolls;  the  lengths  of  all  built  members  l)eyond  centres  of  pin- 
holes; the  spacing  of  rivets  in  flanges  of  stringers,  floor-beams, 
and  chord  members  in  a  general  way,  such  as  "  16  spaces  of 
3"  each,"  or  "3"  spacing  as  nearly  as  may  be";  the  distance 
from  back  to  back  of  opposite  flange  angles  in  all  girders  and 
struts;  the  widths  of  webs  of  all  plate  girders;  the  spacing  of 
stiffening  angles  ;  etc.,  etc.  All  joints  which  are  to  be  planed 
or  faced  should  be  so  indicated. 

Each  sheet  should  have  a  general  and  descriptive  title 
written  in  a  neat  but  plain  style  of  lettering.  The  title  and 
the  number  of  the  drawing  should  be  placed  in  the  lower  right- 
hand  corner. 

A  single  line  drawn  one-half  inch  from  each  edge  of  the 
sheet  should  define  its  maigin,  and  if  a  rather  fine  line  be 
drawn  for  each  boimdary  of  the  tracing,  and  the  sheet  be 
trimmed  just  up  to  these  boundary-lines,  the  blue-prinler  will 
have  a  well-defined  border  to  which  to  trim  his  prints. 

All  lettering  should  be  plain,  but  executed  in  a  neat  and 
workmanlike  manner.  Nothing  adds  more  to  the  appearance 
of  a  drawing  than  neat  lettering.  Special  cure  should  be 
taken  to  locate  all  dimension-lines  so  there  can  be  no  doul  t 
as  to  the  distances  they  are  intended  to  fix.  All  notes  should 
be  written  in  positions  where  they  will  be  easily  noticed,  and 
so  that  they  will  not  interfere  with  the  lines  of  the  drawing. 


340 


1)E    I'ONTIBUS. 


A  set  of  general  notes  sbouUl  be  given  on  each  sheet  of 
details,  specifying  the  kinds  of  material,  the  aizcs  of  rivets, 
the  diameters  of  rivet-holes  before  and  after  reaming,  the 
manner  in  which  all  plates  are  to  he  finished,  etc. 

After  each  sheet  is  pencilled,  it  should  be  checked  carefully 
to  see  that  there  ;ire  no  errors  thereon;  then,  after  the  tracing 
is  finished,  it  must  be  checked  in  detail— if  possible  by  some 
one  who  was  not  concerned  in  its  preparation. 

The  following  standaid  instructions  of  the  author's  to  his 
offlce-assistanls  concerning  the  checking  of  drawings  will  in- 
dicate wliat  such  checking  should  accomplish  and  the  essential 
thoroughuess  thereof : 


OKNKUAL   DETAII,   URAWINOH. 

First.  Go  over  all  drawings  for  the  entire  design  and  see 
that  every  detail  of  the  structure  is  shown  in  a  sullicieul  num- 
ber of  views  to  make  clear  to  the  manufacturers  exactly  what 
is  intended  by  the  designer. 

Second.  See  that  every  detail  has  been  dimensioned  so  that 
it  can  be  readily  laid  out  on  the  working  drawings.  See  also 
that  all  sections  of  coiuiection  angles,  tillers,  etc.,  are  given. 

Third.  See  that  |)roper  ilescriptive  notes  are  given  wher- 
ever necessary  to  make  clear  tlie  leasons  for  any  special  de- 
tails. 

l^-ourth.  Examine  each  detail  and  see  tlsat  every  portion  of 
it  is  strong  enough  to  carry  properly  the  greatest  stress  that  can 
ever  come  upon  it.  Make  sure  that  enougli  rivets  have  been 
used,  and  that  they  are  indicated  to  be  countersunk  or  llal- 
teiied  wherever  necessary  to  [)rovide  proper  clearance. 

Fifth.  In  checking  up  the  packing  at  the  panel  points,  .see 
that  all  members  wliich  are  to  be  i)rouglit  on  to  the  piu  are 
shown,  and  that  a  sutlicient  clearance  has  been  figured  for 
each.  Make  sure  that  all  forked  ends  have  the  requisite 
stronglli,  and  that  diaphragms  between  same  have  been  used 
wherever  necessary.  Check  up  the  bearing  of  each  member  on 
tlie  pin,  and  make  sure  that  plenty  of  rivets  have  been  used 
to  convey  the  stress  from   the  extension-plates    to  the  main 


an( 

ply 

lion 

T( 

lied 

they 

mail 

E 

bers 

the 

Tv 

so  as 

Th 


OFFICE-I'UACTICE. 


341 


member.  Ueinembcr  that  the  stress  to  be  provided  for  at  the 
bottom  of  a  vertical  post  is  not  the  stress  ou  the  post  itself,  but 
ilie  algebraic  s\im  of  the  vertical  components  of  the  stresses  in 
all  diagonals  attaching  to  the  pin  at  the  foot  of  post,  or,  ap- 
proximately and  on  tiie  side  of  safety,  tlie  stress  on  the  post 
plus  one  half  of  a  panel-floor  load.  See  that  uo  bar  diverges 
from  the  central  plane  of  truss  more  than  one  eighth  (|)  of  au 
inch  to  the  foot. 

See  that  tillers  are  shown  and  their  sizes  given  wherever 
they  are  necessary  to  hold  the  members  to  exact  position  ou 
the  pins. 

Check  all  pins  for  the  greatest  bending  moments  coming  on 
them,  (ielernuuiug  the  same  by  combining  the  bending  mo- 
ments in  two  directions  at  right  angles  to  each  otlier. 

Sixth.  See  that  the  centre-of-gravity  lines  of  all  members 
are  shown;  and  where  any  sucii  line  is  not  In  the  central  plaiie 
of  member,  see  that  it  is  located  from  the  side  of  the  section. 

Seventli.  Wherever  a  drawing  is  either  wholly  or  partially 
shown  in  section,  sea  that  the  exact  point  at  which  the  section 
is  taken  is  indicated  in  writing,  and  that  the  section  line  is 
pro|)erly  shown  on  the  other  views  to  which  the  note  refers. 

Kightli.  Compare  all  sections  of  meinl)ers  and  all  leading 
dimensions  with  thosi;  on  the  stress-<iiagram,  and  see  that  they 
corrispond  thereto. 

Ninth.  See  that,  all  slay  plates  and  laeing-bars  are  sliown, 
and  that  the  sizes  for  same  are  given;  also  that  these  sizes  com- 
ply witli  the  requirements  of  the  specifications.  The  inclina- 
tions of  all  lacing-bars  sliould  be  given. 

Tenth.  See  that  all  extension- i)lates  of  forked  ends  are  car- 
ried at  least  six  inches  inside  the  end  stay-plates,  and  that 
they  are  strong  enough  to  develop  the  full  streuglli  of  the 
main  member,  even  though  the  computed  stress  be  small. 

Eleventh  See  tiiat  all  rein  forcing-plates  at  ends  of  mem- 
bers are  so  distributed  as  to  balance  as  nearly  as  practicable 
the  bearing  on  the  two  sides  of  the  main  section. 

Twelfth.  C/ompare  drawings  which  show  the  same  details, 
so  as  to  make  sure  that  all  are  alike. 

Thirleenlh.  See  that  the  same  style  of  detailing  has  been 


342 


IDE    rONTIBUS. 


followed  oil  all  drawings.  Wiiere  several  draftsmen  are 
employed  on  the  same  piece  of  work  there  is  liable  to  be  quite 
a  diversity  of  details,  illiistratlug  the  individualities  of  the 
various  draftsmen  making  them. 

Fourteenth.  When  a  change  is  made  in  any  part  of  a  draw- 
ing, see  that  said  change  is  carried  through  all  the  sheets 
which  a;e  affected  thereby. 

Fifteenth.  See  that  when  any  drawing  or  jiortion  thereof 
is  abandoned  it  is  so  indicated  clearly  throughout  all  the 
drawings. 

Sixteenth.  Check  all  forked  ends  for  transverse  bending, 
and  sec  that  they  have  been  reinforced  wherever  necessary. 

Seventeenth.  Wherever  timber-bolts  are  to  be  used,  see  that 
they  are  plainly  indicated,  that  their  sizes  and  lengths  are 
given,  and  that  washers  are  provided  beneath  all  heads  where 
the  bearing  is  on  the  wood. 

Eighieenth.  See  that  all  screw-ends  of  rodsar^  "»^«!et,  unless 
they  are  to  have  cold-pressed  threads.  See  thuo  all  dingonal 
soils  are  provided  with  proper  adjustments,  and  that  all 
clevis-pins  and  plates  arc  of  proper  strength.  See  that  no 
pins  of  less  than  two  and  a  half  inches  diameter  are  tised,  and 
that  they  are  set  at  least  one  and  one-half  diameters  from  edge 
of  plate. 

Nineteenth.  See  that  each  sheet  is  provided  with  general 
notes,  as  follows  . 

A.  Kinds  of  material  to  be  used  throughout  the  structure. 

B.  Diameters  for  rivets. 

C.  Sizes  of  rivet-holes  before  and  after  reaming. 

D.  Manner  in  which  the  edges  of  all  web-plates  are  to  be 
finished. 

E.  Wiuxt  ends  arc  to  bo  faced  and  what  are  not. 
Twentieih.  See  that  all  notes  are  written  in  good  English, 

that  all  words  are  spelled  correctly,  and  that  they  express 
exacily  what  is  intended. 

Twentytlrst.  See  that  each  drawing  is  provided  with  proper 
titles,  that  it  is  numbered  correctly,  that  the  scale  or  scales 
are  indicated,  and  that  I  lie  name  of  the  draftsman  and  date 
of  completion  of  drawing  ai'e  given. 


OFFICK-FIIACTIGE. 


343 


Twenty  second.  See  tlmt  the  drawings  scale,  and,  if  tlicy 
do  not,  make  a  note  sa3ing  that  the  diinensions  written  on  tlie 
drawings  are  to  be  followed  in  preference  to  the  scale  win  re 
tiiere  is  any  discrep'incy  bctwien  the  two. 

Twenty-thiril.  In  slioit,  check  over  all  details,  diinensions, 
sectOns,  and  notes  given  on  tlje  drawings,  so  as  to  make  sure 
that  cverylliing  is  in  strict  accord.mc  e  with  the  specifications 
aud  with  the  data  furuished  for  the  structnre. 


SnOI'   DHAWINOS. 

First.  Make  sure  that  the  sections  and  details  conform  in 
every  particular  with  those  given  on  the  general  detail  drawings 
and  stress  diagrams,  excepting  in  minor  points,  where  sligiit 
ciianges  may  be  made  to  facilitate  the  work  in  the  shop.s,  pro- 
vided, of  course,  that  such  alieratious  do  not  in  any  way  imixiir 
the  strength,  dunibility,  or  appearance. 

Second  Check  over  all  tield  connections  to  see  that  there 
are  no  rivets  which  are  so  located  that  they  cannot  be  satisfac- 
torily driven  in  the  tield. 

Third.  See  that  all  members  have  proper  clearances  at  panel 
points,  and  that  all  rivet-lieads,  wherever  necessary  to  provide 
such  clearances,  are  countersunk  or  flattened. 

Fourth.  Check  over  all  lengths  of  members  and  rivet- 
spacing  for  field  connections  to  make  sure  that  the  holes  will 
match  in  tl»e  held. 

Fifth.  ClK'ck  over  all  bills  of  material  to  see  that  the 
proper  number  of  pieces  liave  been  ordered,  and  that  they 
are  of  proix-r  sections  and  lengths. 

Si.vlh.  Always  have  the  shop-drawings  sent  to  the  ollice  in 
duplicate,  and  check  up  the  two  sets,  retaining  one  set  in  the 
olHce  and  returning  the  otiier  set  with  corrections  or  approval 
marked  thereon.  Where  drawings  are  returned  to  shops  with 
corrections  marked  on  them,  revised  prints  must  be  scut  for 
approval  before  the  work  is  put  into  the  shops. 

It  is  often  necessary  to  make  changes  on  a  tracing,  and  in 
doing  !=o  great  care  should  be  exercised,  otherwise  a  drawing 
wliich  has  cost  consi  !erai)le  time  and  money  may  be  ruined. 


:iu 


DE   P0NTIBU3. 


For  making  slight  erasures,  n  very  sharp  kuife  skilfully  used 
will  be  found  effective,  as  it  can  be  so  munipuluted  as  to 
affect  nothing  but  the  parts  to  be  erased.  Another  expedient, 
where  only  a  slight  erasure  is  to  be  made,  is  to  use  a  tliiu 
sheet  of  celluloid  or  durable  cardboard,  in  which  are  cut 
small  holes  corresponding  lo  the  work  to  be  changed.  This 
sheet  is  laid  on  the  drawing  so  that  a  hole  comes  over  the  part 
lo  be  erased,  then  a  sjiud-eraser  is  rubbed  over  the  hole,  and 
nothing  is  damaged  except  the  portion  which  is  changed. 


FILING   DRAWINGS,    CALCULATIONS,    SPECIFICATIONS,    ETC. 

In  the  course  of  a  few  years'  practice  the  office  records  of  a 
consulting  engineer  grow  to  such  proportions  that,  unless 
some  systematic  niclhod  of  filing  and  indexing  them  be 
adopted,  it  is  impossilile  lo  refer  thereto  without  a  great  deal 
ot  delay  and  annoyance.  The  filing  of  calculations  and  speci- 
fications is  a  comparatively  easy  matter,  but  to  keep  an 
accumulating  lot  of  drawings  in  good  shape  for  ready  refer- 
ence is  by  no  moans  sucii.  During  the  time  that  the  author 
has  been  engaged  in  active  practice  sevend  methods  have  be(!n 
employed  for  filing  tracings.  One  great  difflrulty  with  the 
earlier  drawings  was  that  they  were  of  varying  dimensions, 
some  as  large  as  forty-two  inches  by  ninety-six  inches,  and 
otiiers  l)elonging  to  the  same  set  as  small  as  eighteen  iiiclies 
.square.  At  first  large  cases  of  drawers  were  used  for  laying 
out  the  tracings  flat,  each  tracing  being  .stamped  with  num- 
bers designating  the  lot  and  drawer  to  which  it  belonged,  and 
an  index  being  kept  of  all  drawings  recording  the  numbers  of 
the  lot  and  drawer.  The  objections  to  this  method  were  that 
the  smaller  drawings  got  lost  among  the  larger  ones,  thus 
often  necessitating  a  complete  overhauling  of  an  entire  drawer 
to  find  a  tracing,  and  it  was  impossible  to  keep  the  large 
drawings  from  beconnng  folded  and  cracked  at  the  edges  and 
corners. 

Later  it  was  deemed  advisable  to  bind  each  set  of  drawings 
together  with  patent  fasteners  along  one  end,  but  this  methotl 
was  soon  abandoned,  owing  to  the  difliculty  encountered  in 
getting  out  tracings  for  blue-printing  and  reference. 


OFFICR-PRAOTICE. 


345 


The  method  of  liiyhig  tlie  tmciugs  flul  iu  dniweis  wiis  aban- 
doned, aud  they  are  now  flhul  iu  oaidljoaid  tubes,  thirty 
indies  long  and  four  inches  in  diameter,  with  tightly  tilling 
covers  of  the  same  material.  Eaeli  tnbc  lias  on  tlic  cover  its 
index  niimlier  and  a  lype-writlen  list  of  the  tracings  it  con- 
tains. The  tracings  are  roiled  in  small  bundles  of  four  or 
tive,  and  each  bunch  is  held  to  small  diameter  with  a  rubber 
band,  to  which  is  attached  a  tag  giving  the  number  and  title 
of  each  of  the  sheets  contained  in  the  roll.  Five  rolls  are 
placed  in  each  tube,  making  a  total  of  from  twenty  to  tweuty- 
tive  tracings  per  tube.  The  tracings  should  not  be  rolled  so 
closely  that  they  will  become  creased. 

An  index  is  kept  of  all  tubes,  g'ving  their  numbers  aud  the 
titles  of  the  drawings  contained  in  each  ;  aud  there  is  iu  addi- 
tion an  alphabetical  index  of  the  drawings. 

The  tubes  are  set  in  cases  with  ilieir  covers  exposed,  and  are 
so  arranged  that  any  lube  can  be  easily  reached  or  removed 
from  the  case  if  necessary. 

Copies  of  all  shop-drawings  arc  also  kept  on  file  for  refer- 
ence. The.se  are  put  in  larger  tubes,  aud  as  there  is  never 
any  necessity  for  separaiing  a  set,  as  is  the  cast;  with  the 
tracings,  each  set  is  bound  together  when  complete.  Tlie 
shop-drawings  are  all  inclu  led  in  liie  two  indices. 

The  specilicat ions  and  ciilcuiiilions  an;  kepi  in  tiling  cases 
prepared  especially  for  them,  and  both  .-ire  indexed.  These 
CI  sea  consist  of  a  series  of  small  shelves  about  one  and  a  half 
inches  apart,  each  shelf  being  numbered. 

When  a  set  of  calculations  is  complete,  the  sheets  are  all 
bound  together  iu  one  book  willi  removable  fastenings,  so 
that  Ihey  can  be  easily  separated  when  il  is  necessary  to  dis- 
tribute tliem  among  several  draftsmen.  These  sets  are  all 
numbered  with  the  numbers  of  the  .shelves  on  which  they  are 
to  ')o  tiled. 

In  indexing  all  work  every  article  should  be  indexed  under 
as  many  headings  as  practicable. 


346 


DE   I'ONTinUS. 


OFFICE   MATERIALS. 

All  calculntioii -blanks  sliould  be  of  an  extrugoodqimlity  of 
paper,  capable  of  witlistaiiding  a  great  deal  of  erasing  and 
scratching,  which  is  ofien  necessary  in  niaking  sketches  for 
d(  tails.  The  tracing-cloth  should  be  of  the  best  cpiality,  as  it 
is  impracticable  to  make  a  good  drawing  on  poor  cloth.  The 
best  brand  that  the  author  lias  ever  used  is  ihe  Imperial. 

Powdered  chalk  or  talcum  should  be  rubbed  over  the  sur- 
face of  the  tracing-cloth  to  make  it  lake  the  ink  unit'orndy. 
Pencil-marks  and  dirt  can  be  easily  removed  from  a  tracing 
by  moistening  a  towel  in  benzine  aiul  washing  the  surface  of 
the  clotii  with  it.  If  a  good  (pialily  of  ink  be  used  it  will  not 
be  affected  by  such  washing. 

There  are  many  liquid  India  inks  in  the  market,  but  none 
of  them  will  give  quite  as  good  results  as  will  the  genuine 
stick  ink  when  properly  ground;  nevertheless,  except  for  very 
fine  work,  the  former  are  preferable  on  account  of  the  saving 
of  time  which  they  effect.  Iliggins*  water-proof  ink  is  the 
most  salisfa(;tory  which  has  yet  been  tried  in  the  author's 
office, 

A  gootl  quality  of  brown  detail  paper  is  very  essential,  for 
there  is  in  all  kinds  of  detailing  a  groat  deal  of  erasing  to  be 
done;  and  time  is  always  saved  by  using  good,  tough  paper 
that  does  not  rough  up  by  having  an  eraser  used  upon  it. 


CONCLUSION. 

In  concluding  this  chapter  on  "  Office  Practice"  the  author 
desires  to  again  call  the  reader's  attention  to  the  necessity  for 
adopting  the  most  systemaMc  methods  possible  for  doing  all 
kinds  of  work,  keeping  all  kinds  of  records,  and  filing  all 
kinds  of  accumulated  material.  As  soon  as  a  large  piece  of 
work  is  finished  a  thorough  systemizalion  should  be  made  of 
the  knowledge  obtained  in  making  both  the  design  and  the 
various  calculations,  so  that  the  office  force  shall  he  able  to  use 
tlie  same  to  the  best  possible  advfintage  when  starting  on  an- 
other similnr  piece  of  work.     And    whenever  there   is  &i)y 


OFFICE-PRACTICE. 


347 


spnre  thne  In  the  office  for  uny  of  the  employees  it  should  be 
devoted  t(i  (icciimuliaifjg,  digesting,  and  putting  in  conveulent 
form  for  use  the  results  of  previous  investigatious,  and  to  do- 
ing  such  work  as  tubulating  and  recording  on  diagrams  the 
weigiits  of  metal  per  lineal  foot  of  span  for  bridges  of  all 
kinds. 

Finally,  in  bringing  this  little  treatise  to  a  close  the  author 
feels  that  he  cannot  do  better  than  to  repeat  from  Chapter  II 
the  following  principle  •,  "The  systemization  of  all  that  one 
does  in  connection  with  his  professional  work  is  one  of  the 
most  important  steps  that  can  be  taken  towards  the  attainment 
of  success. " 


TAHLli   I. 


551 


Table  I. 


COEFFICIENTS    OF  IMPACT   FOR  RAILWAY   HRIIMJES. 

400 


1  =  Inifiat't. 
L  ~  Length  of  Span  in  Feet. 


/  = 


TjOO 


L 

/ 

L 

1   50 

I 

L 

/ 

1 

0.7984 

0.7273 

99 

0.6678 

S 

0.7968 

51 

0.7260    i 

100 

0.6667 

3 

0.7952 

52 

0.7247    i 

105 

0.6612 

4 

0.793C 

53 

0.7233    i 

110 

0.6.557 

5 

0.7921 

.54 

0.7220 

115 

0.6,504 

6 

0.7905 

55 

0.7207 

120 

0.64,52 

7 

0.7889 

1   56 

0.7194 

125 

0.6400 

8 

0.7874 

57 

0.7181 

130 

0.6349 

9 

0.7a58 

58 

0.7169 

135 

0.6299 

10 

0.7843 

59 

0.71.56 

140 

0.62.50 

11 

0.7828 

1   60 

0  7143 

145 

0.6202 

\i 

0.7812  _ 

61 

0.71.30 

1.50 

0.61.54 

13 

0.7797 

62 

0  7117    ! 

155 

0.6107 

14 

0.7782 

63 

0.7105 

160 

0.6061 

16 

0.7767 

64 

0  7092 

165 

0.6015 

16 

0.7^52 

65 

0.7080 

170 

0.5970 

17 

0.7737 

66 

0.7067    1 

175 

0.,5926 

18 

0.7722 

0.7055 

180 

0.5883 

19 

0.7707 

!   68 

0.7W2 

185 

0..5839 

20 

0.7092 

i   69 

0.7030 

190 

0.5797 

21 

().7()78 

1   7C 

0.7018 

195 

0.5755 

22 

(1.7063 

71 

0.7005    i 

200 

0.,5714 

23 

0.7648 

72 

0.6993 

210 

0.,5ti34 

24 

0.76:y 

73 

0  6981 

220 

0..5556 

25 

0.7019 

74 

0  6968 

230 

0.5480 

26 

0.7605 

75 

0.6957 

240 

0.5405 

27 

0.7590 

76 

O.C.i)44 

250 

o..5;i;i3 

28 

0.7576 

77 

0.0932 

260 

0  .5263 

29 

0.75C1 

78 

0.6920 

270 

0.5195 

30 

0.7.047 

i   79 

0.6908 

280 

0.5128 

31 

(».75;« 

1   80 

0.6897 

290 

0.,5063 

3J 

(t.7.M9    1 

81 

0.6K85 

300 

0.5000 

33 

0.7505 

82 

0  6873 

325 

0.4848 

34 

0.7491 

83 

0.6861 

3.50 

0.4706 

35 

0.7477    : 

84 

0.6849 

375 

9.4.571 

36 

0.7463    j 

85 

0.68:18 

400 

0.4444 

37 

0.744U 

86 

0.6826 

450 

0.4211 

38 

0,7435 

87 

0.6814 

.500 

O.KKX) 

39 

0.7421 

88 

0.6803 

5,50 

0.3810 

40 

0.7407 

89 

0  6791     ! 

600 

0.31)36 

41 

0.7394 

90 

0.6780    1 

650 

0..34;8 

42 

0.7380 

.   91 

0.6768 

700 

0.3*33 

43 

0.7367 

!   92 

0.6757     1 

750 

0.3200 

44 

0  735.1 

93 

0  6745 

HOO 

0.3077 

45 

0.73:W    ' 

94 

0.6734 

850 

0.2963 

46 

0.7326 

95 

0.6723 

900 

0  2857 

47 

0.7313 

.   9(i 

0.6711 

9,50 

0.27,59 

48 

(1.7^)9 

97 

0.6700 

1000 

0.2667 

49 

0.'i286    j 

1 

98 

0.6689 

1 

1 

352 


1)K    I'OXTIHUS. 


Taiu.k  II. 


COEFFICIENTS  OF  IMPACrr  Fuli  IIIUHWAY    HUllXJES. 

]0<) 


/  =  Impact. 

L  —  LeiiK'li  of  Span  in  Feet. 


L  -\-  ISO' 


L 

/ 

L 

1 

0.f.ti-.i:^ 

50 

2 

o.ti.jry 

51 

3 

0.0536 

58 

4 

0.tJ494 

Ki 

6 

O.M'o-i 

54 

6 

0  6410 

1       ^'5 

7 

0.6369 

56 

8 

0.6389 

57 

9 

0.6-.iS9 

f>8 

JO 

0.6'jr)l) 

59 

11 

0.6811 

60 

18 

0.6173 

61 

i;< 

0.6134 

i;8 

14 

(1.6098 

63 

\h 

(I  6061 

64 

16 

0.6084 

r>r< 

17 

0.59f8           1 

60 

W 

0..')»58           1 

67 

10 

0.5917           1 

68 

30 

0..5888 

69 

SH 

0.5848 

70 

0..'^i8!4 

71 

iS 

0  5780 

78 

«4 

0..5747 

18 

w 

0..')714 

74 

10 

0.5688 

75 

«? 

0.5650 

76 

m 

0..')618 

77 

•» 

()..->.5H6 

7H 

m 

O..Vm6 

79 

81 

0.5585 

80 

an 

()..5495 

1       HI 

8B 

0.5405 

88 

S4 

0  .'■485 

83 

86 

(l.540."> 

84 

m 

0.r.3r6 

85 

«7 

(1  .')3»H 

80 

8B 

().'.5:!19 

87 

8» 

0.5891 

88 

40 

0.53C3 

89 

41 

0.5286 

00 

4« 

0  580M 

01 

48 

0.6I8I 

08 

4i 

0.51.5.') 

03 

45 

0..518H 

04 

46 

0.5108 

95 

47 

0.6076 

90 

48 

0.5051 

97 

49 

0  r^i& 

.      08 

0.. '■)()( )() 
0.1'.(75 
0  I!I51 
0.4986 
0  4908 
0.4K78 
0.4K51 
0.4S31 
0.4 808 
0. 1785 
0.(768 
O.K3;l 
0   1717 

().4t;;),") 
0.  ir7;i 

0.  ICi.')! 
O.KWiO 
0.4608 
0.4.^87 
0.4.')66 
0.4516 
0.4.'A'5 
0  4.505 
0  4181 
0.1161 
0  4444 
0.4485 
0  44<>:i 
O.IW. 
0. 1367 
0.  1348 
0.4389 
0.  1310 
0.4898 
0.4874 
0  4855 
0.4-.'87 
0  1819 
0  180.' 
0.41S4 
0.4167 
0.4149 
0. 1131 
0.4115 
0.4098 
0.4088 
0.406r> 
0.4049 
0.4033 


90 

0.4016 

100 

(l.4(l(X) 

105 

0.:!!t88 

no 

0  :i846 

115 

0.3774 

K'O 

0.370.4 

185 

(1.36.36 

130 

0  .■i.-.7l 

i:!5 

0.3.509 

110 

0  :;M8 

145 

(1  3389 

1.50 

0  ;i.i.33 

1.55 

('  3879 

Mil) 

(>..3.'-.'6 

l6-> 

ii.-:i';5 

170 

(I..il85 

17.-. 

0.. 3(177 

180 

(1.30,30 

185 

(1  •.'985 

190 

0.8911 

195 

0.8899 

800 

0  8H.-)7 

810 

0.8778 

3-.>0 

0.8703 

830 

0.86.38 

840 

0.8.564 

850 

0  8.50<» 

860 

0.8 139 

870 

0.-.'.l8I 

880 

0  8.i86 

-.90 

(i.8-.';3 

3(10 

0  ■:-:-:'i 

385 

0.8105 

:i-.o 

0  8(K)0 

375 

0.1',»(I6 

100 

0.1818 

4.50 

0.1667 

.500 

0.15,39 

5,50 

0.1489 

600 

o.vm 

650 

0  18.50 

700 

(t.ll76 

7,50 

0.1111 

800 

0.10,53 

8.50 

0.1(100 

900 

(I.(l',i58 

9.50 

0.11909 

1000 

0.0870 

fSftu^ 


TAr.i.i;  III. 


353 


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5'. 


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TABLE  VtU 


35 


Oi 


[ 


^ 


?5 


Table  VII. 

CENTRIFUGAL  FORCE  IN  PERCENTAGES  OF  LIVE   LOAD. 
V  X  100 


C.F.= 


32. a  X  ft" 


V  =  Velocity  in  feet  per  second.    R  =  Radius  in  feet. 


De- 

Velocity  in  Miles 

per  Hour. 

Kfee. 

10 

10 

20 

!?o 

30 

:t.) 

40 

60 

60 

1 

0.12 
0.23 

0.20 
0.53 

0.46 
0.'.3 

0.73 
1.40 

1.05 
2.10 

1.13 
2.80 

1.87 
3  73 

2.91 

5.8-.' 

4.20 

o 

i<.U) 

3 

0.35 
0.47 

0..58 

0.7:1 

1.05 
1.31 

1.40 
1.86 
2.3  J 

2,19 
2.92 
3.05 

3.15 
4.20 

5.25 

4.28 
571 

5.00 

1     S   .4 
11.65 
14. 57 

12  .59 

4 

7.40 
9.33 

10.78 

.>> 

7.14 

20.99 

6 

0.70 
0.8-,' 

i.,5; 

1   HI 

2.79 
3  20 

4.. 37 
5.10 

0.30 

8. 50 
9  99 

11.19 
13.05 

17.48 
20  39 

25.17 

r 

7  31 

','9.30 

8 

0  m 

2.10 

3. 7! 

5.H2 

8.39 

11.42 

14.91 

23.30 

»i.Rr> 

9 

1.10 

2.36 
2.02 

4.19 
4.00 

0..55 

9.43 

10.48 

12.84 
14.'J6 

16.77 
18.03 

20.80 
29.11 

37.74 

10 

7.2^ 

41.92 

11 

1 .28 

2.  HO 

5.12 

8.00 

11.52 

15.68 

20.49 

32.01 

IJ 

1.41) 

3.11 

5  58 

8.73 

12.59 

17.11 

22.35 

31.89 

13 

1..51 

3.40 

0.0') 

9  45 

13.61 

18  53 

24.20 

11 

1.63 
1.74 

3.00 
3.92 

6.M 

10.17 
10  90 

14.65 

15.70 

19.94 

2l.;i0 

:.'6.05 
27.90 

1.5 

0.9; 

16 

1.86 

4.18 

7.43 

11.02 

16.73 

22.77 

17 

1.98 

4.44 

7.90 

12.31 

17.77 

84.19 

18 

2.09 

4  70 

8.30 

13.00 

IS. 81 

25.60 

19 

a.  21 

4.96 

8.8-.' 

13, lO 

19.85 

27.00 

ao 

2.32 

5.22 

9.28 

14.51 

20.88 

28.42 

21 

2.44 

5.48 

9.74 

15.22 

21  91 

29  82 

aa 

2.55 

5.74 

10.20 

15.94 

82.95 

81.23 

23 

2.06 

5.99 

10.05 

10  65 

23.97 

32.63 

24 

2.78 

6.25 

11.11 

17.37 

25  00 

34.02 

2.5 

a.H9 

0.51 

11.57 

18  08 

20.02 

:i5.42 

ac  . 

3.01 

;M2 

6  70 

12.02 
12  17 

18  79 

nt..5<» 

27.05 
28  07 

.30.81 
38  20 

27 

7.02 

28 

•■i'£i 

7.2: 

12.93 

20.20 

29.09 

39.59 

29 

8  34 

7.N3 

1 3.-37 

20.91 

30.11 

40  97 

80 

3.40 

7.78 

13.8.1 

21.0J 

3  I.I-.' 

42  3.5 

31 

.s  y, 

8.(« 

1t.2S 

22.32 

12  13 

13  73 

82 

S  «8 

8.29 

14.73 

23  ai 

.33.15 

45  11 

83 

3.M> 

8  54 

15.18 

23.^2 

31,10 

40.49 

34 

3.91 

s.;9 

15.62 

21.42 

35  111 

17  8,') 

35 

4.02 

9.04 

10.07 

25.12 

30.15 

49  20 

36 

4.13 

(1,2^ 

10.51 

25.81 

37.10 

.5<t..57 

37 

4  -.'4 

v>  .M 

10.95 

20  .50 

3><.15 

51.9-.' 

38 

4  » 

S.79 

IT.  39 

•-'7.19 

39.14 

.53  27 

39 

4  40 

10. (M 

17  84 

27  S8 

40  U 

51.02 

40 

4.57 

10.28 

18.27 

28.. 57 

41   12 

55.97 

4! 

4.68 

10  53 

18.71 

29.24 

42.10 

57.  .30 

48 

4  70 

10  77 

19  15 

29.93 

43.09 

58  (iO 

43 

4.!K) 

11.02 

19.,5S 

30.01 

41.08 

59  99 

44 

5. 01 

11.20 

20.01 

31.29 

45  04 

01.29 

46 

5.11 

11.. 50 

20. 14 

31.95 

40  (X) 

02  01 

46 

5.2-.' 

11.71 

20.87 

.32.03 

40  97 

47 

5.:« 

11  90 

21.30 

:i3..30 

47  95 

48 

5.41 

12.23 

21.73 

33.98 

48  92 

49 

5.54 

12   10 

22.15 

34,03 

49.85 

SO 

5.05 

12.';i 

22.58 

.35.30 

.'W.82 

NoTK, — The  stepped  line  shows  the  Uniitiiig  percentajjes  for  a  super* 
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Table  ix. 


3f)l 


Tabi.k  IX. 

BENDING   MOMENTS  ON   PINS. 


Moments  In  In.- 

MdMientM  In  In  • 

1 
t 

Moments  ill  In.- 

a 

H>8.  for  Fibre 

c 

lbs   fnrFitnv 

Il)s.  for  Fibre 

e 

•a 

Stress  of 

£ 

0 

Sti-ess  (if 

0 

8tre.>i8  of 

0 

E 

ar.ooo 

;i5,100 

27.(XX)      35,  UX) 

27,0(¥) 

35,1(X) 

i 

lbs.  per 
sq.   n. 

ll)s.  per 
Rq.  in. 

.2 

ll)M.  per 
sq.  in. 

lbt<.  per 
S(|.  in. 

c3 

2 

lt)H.  per 
S({.  in. 

li)s.  per 
sq.  In. 

"Uf 

ai'.HX) 

27570 

801300 

1119700 

ii»6" 

4104r><H) 

5413900 

•iht 

'JSTiOO 

332(X» 

1 

!XI9I(X) 

118I8(XII  11-H 

4»XHXH) 

.5.51MHXX) 

«^-4 

*WH) 

;)9:«X) 

7'ii 

!).").S,S(H) 

124640(1 

11% 

'i439:i(K) 

5771100 

■-'•)n 

x>rA)() 

4f.-.'00 

''4 

KIIOKK) 

1313100 

12 

4.580.500 

59.547(K) 

•4 

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•% 

1IH;3.)(K)'    1382;i(X) 

12^ 

4VJ50tK) 

61425<X) 

-% 

4;9()0 

02300 

7U 

IllWiOOl    14M8(X) 

12^4 

4W727(X) 

6834.5(X) 

55100 

71700 

T')^ 

H7.")100 

1527.500 

12?6 

.502.'Xi()0 

t;5;io7(x) 

63()()U 

HI  900 

7.Vi 

1233!KX) 

1(!04100 

12U, 

5i;72(»0 

6730400 

•1 

7I6(K) 

93100 

"^/k 

12D4.'iOO 

lfi827(X) 

I25A 

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6936100 

3V6 

80800 

105000 

8 

13,-)720O 

17»544(H) 

1234 

549KMM) 

7142200 

m 

91000 

118300 

m 

1421800 

184H3(X) 

1'-'% 

.';057300 

7354500 

101900 

1.J2500 

814 

1488.500 

1935100 

13 

5823000 

767070C 

31? 

1130(X) 

1477(X) 

t*% 

1557000 

2024100 

I3>4 

5il932(X) 

7791200 

•'% 

I'.'ftWO 

1642(X) 

1627S00 

2116100 

13^4 

6I66(I(X) 

8015800 

1.39800 

181700 

^7H 

17(X)70() 

22109(.0 

mi 

fi342300 

824600(1 

15ia00 

200,t00 

S^/^ 

1775700 

23aS400 

131^ 

6.521700 

847820(1 

i 

1C96(X) 

220.500 

18.')3(XX) 

2408900 

13»Ji 

6704600 

8716000 

'>Ht 

186100 

24  mx) 

9 

1932300 

2518100 

13.% 
13% 

6890900 

8968200 

1^ 

203500 

264500 

9)4 

2013!XX) 

2618100 

7080500 

9204700 

•v'2-,'000 

2H80(X) 

!44 

2097900 

2727.300 

14 

72738(X) 

9455900 

■i?! 

2415(K) 

314000 

9% 

2184:^(X) 

2839600 

'•l'/^ 

74700(X) 

9711000 

a62200 

310900 

9UJ 

9W 

2272!XX> 

29.54800 

ml 

';6707(X) 

9971900 

284100 

369300 

2363(i(X) 

3072700 

7874(KX) 

10236200 

307100 

3992(K) 

9% 

2157000 

3194100 

141^ 

8081 1(X) 

10.505400 

5 

331 3(X) 

430700 

2.V.2.5(X1 

3318300 

|H% 

82920*  X) 

10779(XX) 

•"iJ^l 

85«S(H) 

463fKX) 

10 

2(i.')0!KX1 

;W4(i2(H) 

jl4?i 

ml 

85(XM0O 

II058;«X) 

^ 

3S3r.O0 

498700 

loki 

275 1«X) 

3.576700 

87245(X) 

11841900 

411000 

535100 

1014 

2854400 

3710700 

15 

89462(X) 

1I(»0I00 

•'>^ 

441000 

573:«X) 

IO?<i 

•i'MWiOO.  ;W8400l 

15^ 

9S712(X) 

12832600 

r^H 

471800 

ci:«oo 

lOK. 

;i(H)S(i(X) 

3989200 

le 

10H,572(X) 

14114400 

l! 

504000 

C,')5200 

I0->8 

317!)."i00 

4l3-i400 

16^ 

11907300 

1.5479.500 

587.^)00 

698700 

loa. 

3292900 

4280S00 

17 

13022!X)0 

169298(X) 

U 

572500 

744300 

10% 

3409000 

4432500 

I'l''^ 

142063(X) 

18468:.'00 

«^1 

(109200 

792000 

11 

3.'^.28100 

4586500 

18 

154.591(X) 

20096800 

tiM 

647100 

841200 

ll'-H 

3649!XX) 

4714900 

IS^ 

16783500 

21818600 

«% 

68C700 

892700 

HM 

3774300 

4906600 

19 

1  SI  81 300 

23635700 

0^ 

728000 

940400 

ii«/h 

;W01500 

5072000 

^'■H 

I!M;.546(X) 

25.55 10(X) 

c?*i 

7707(X) 

10019(X) 

111^ 

4031400 

5240800 

90 

21205800 

27567500 

6^4 

815300 

1059800 

Note.— 270(K)  lbs.  is  tlie  allowalile  stress,  exclnilinp  wind. 
35100  lbs.  is  the  allowable  stress,  including  wind. 


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TEST  TARGET  (MT-3) 


4 


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Corporation 


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33  WEST  MAIN  STREET 

WEBSTER,  N.Y.  MSBO 

(716)872-4503 


If  4 


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bG2 


1)E    I'ONtlDL'S. 


Table  X. 

BEARING  ON  PINS. 


BeaiiDK. 


Diam. 

of  Pill. 


82000  Lbs. 
per  Sq.  In. 


44000 
4(I80U 
49.500 
.5-.i300 
•VjOOO 

.•irsoo 

OOfiOO 
6.S:^00 
6(!000 
6>*8()0 
Tl.'iOO 
74^00 
:700t) 
7DH00 
Hg.'sOO 

avioo 

88000 
90H00 
9.3.500 
06.300 
90000 
101800 

i04r)00 

107300 
110000 
118800 
11.5.500 
118.M0 
181000 
18.«0() 

129.300 
138000 
134800 
137500 
140300 
14.3000 
146800 
14^500 
161300 


88600  Lbs. 
per  Sq.  In. 


57800 
00800 
04400 
67000 
71.500 
75100 
78700 
88800 
85800 
80400 
93000 

m»oo 

100.00 
103700 

lO'.doo 

110800 
114400 
118000 
181600 
125100 
188700 
1.38:300 
185000 
130400 
14:iOOO 
140600 
1.50800 

i,5;jroo 

1.57300 
160000 
161500 
168000 
171U00 
17.5800 
178a00 
188300 
186000 
189500 
103100 
106600 


Dial.  I. 
of  Pin. 


Bearing;. 


28000  Lbs. 
per  Sq,  In 


154000 
156800 
159.500 
108.300 
165000 
167800 
17a>00 
173800 
176000 
178800 
1MI500 
184:300 
187000 
180800 
108500 
lOoSOO 
108000 
800800 
203500 
806300 
209000 
811800 
214.500 
817:300 
220000 
888800 
88.5.500 
888-300 
8:31000 
8:33800 
8.36.500 
••:30.3OO 
248000 
841800 
847,'>00 
8.50:300 
25:3000 
855S00 
8.58500 
861300 


88600  LbR, 
per  Kq.  In. 


800800 
80:3SOO 
807100 
810000 
814500 
818100 
88K00 
825810 
288«(K) 
232400 
886000 
239500 
84-3100 
816700 
850:i00 
85:3800 
8.57 1(K( 
261(K)0 
864IX)0 
268100 
871700 
27.5.300 
278900 
888400 
886000 
889600 
893200 
8116700 
300300 
303000 
;tO7.')00 
311000 
314000 
.318.'00 
:381S00 
3.'.5:300 
388U00 

3;38r>oo 

.336100 
;330600 


Note.— 82000  lbs.  per  sq.  in.  is  the  allowable  stress  excluding  wind. 
28600    "      ' "  "      including      " 


Table  xi. 


363 


Table  XI. 

INTENSITIES    FOR    FORKED    ENDS    AND    EXTENSION-PLATES 
OF  COMPRESSION  MEMBERS. 

Formula:  P  =  10000  -  Wr. 


I 

1 

P 

1 

9700 

•1 

WOO 

3 

9100 

4 

H800 

5 

8500 

6 

8200 

7 

7900 

8 

7600 

9 

7300 

10 

7000 

P 

I 
t 

P 

6700 

31 

3700 

MOO 

22 

8400 

6100 

23 

3100 

6800 

24 

2800 

5500 

25 

2500 

5S0O 

26 

2200 

4900 

27 

1900 

4600 

28 

1600 

4300 

29 

Mm 

4000 

30 

1000 

304 


Di:  Pont  I  BUS. 


i 

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» 


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09b,     ojb,     fiotc,     cob,     cqBc,     cnb. 


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TABLE   XIII. 


36d 


M 


253 


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S  i(  6t  9(  of  K  $  c> 


sraf^f  jj!^-^^  sr:^^  gr^sr  sr:i?3r  jsaKS 


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¥     .2 


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^owi.Qoo.o-2;5Si£SU22?5?J?{|l55^S 


366 


DE    PONTIBUS. 


cn 
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03 
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TABLE  XV. 


M 


367 


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^AiAS;dl-i>Q& 


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368 


DE    I'ONTIBUS. 


Table  XVI. 

INTENSITIES  OF  WORKINd-STRESSES  FOR  VARIOUS 
MATEKIALS. 


TENSION-STRESSES. 

Eye-bars IHOOO  lbs.  per  sq.  in. 

Shapes 18000   "        '     "    " 

FlaHK»H  of  door-beams  and  striiif^ers  (countini;  in  % 

of  web) 14000  • 

Hip  vert lcal«  (eye-li(irs) IBOOO  ' 

"  "         (Hhaitet*)  and  liiiiiffer  plates  *  HIKX)  ' 

Adjustaltle  menilwi's,  soft  steel  .        .   ......  ItKKX)  ' 

•'  ••  wmiiKht  iron i:«00  • 

Lateral  rods      18000 

siiapes ItJOOO  • 


CUMPKEMSION-8TRK8HK8. 

Top-ohords 18000  lbs.  -  70- per  sq.  in. 


Inclined  end  posts 

Intermediate  posts  and  KiibdliiK<>i>als 
I^Ateral  struts  (no  impact  for  wind  loads) 
Columns  of  viaducts  (fixed  ends) 


18000  lbs.  -  80^ 
r 

16000  lbs.  -  80- 

r 


16000  lbs. 
16000  ;bs. 


J 


»(        II        ti 


II         II         li 


*■         II        il 


60- 
r 

00-    "    "    •• 

r 


{I  =  unsupported  length;  r  -  radius  of  gyration,  both  in 
same  unit.) 


Forked  ends  and  exleusion^plates. 


10000  ibs.  -300^ 


i«      li      fti 


(/  =  length  in  inches  from  centre  of  pinhole  to  first,  rivet 
beyond  point  wlierc  full  section  of  member  beting; 
(  =  thickness  of  plate.) 

Rollers,  allowing  for  impact,  static  load (XKkt  per  lin.  in. 

*'  "         •'  moving  load SOOd 

(d  =  diameter  of  rollers  in  inches.) 


.i    .%    It 


SHKAKINO-STRKSSKS. 


Webs  of  plate-girders,  medium  steel,  net  section  .      10000  lbs.  per  sq,  in. 
Pinsandrivets 12000    '*     "     ''    " 


BBNDtNG-STHBSSBS. 

Extreme  fibre  of  rolled  sections  of  medium  steel, 

impact  included 16000  Ibs.  per  sq.  in. 

Extreme  fibre  of  timber  beams,  impact  included  .   .     2000    "      "     "    " 


♦  Increase  net  section  through  eye  50  per  cejit  ovpr  tliat  of  bo<1y  ojf 


TABLE   XVI. 


369 


Table  X\ I— (Continued.) 

INTENSITIES  OF  WOKKIN(J-H  PRESSES  FOR  VARIOUS 
MATERIALS 


Impact,  railway  bri(JKeK,    /  = 


Iiiipaci,  high  auy  bridKusii,  I  =: 


400 
L  +  500" 

100 


L  +  150" 
(L  =:  Length  in  feet  uf  span.) 

For  reversiog-BtreHKfH  tlgiire  tli«  arean  ret|iiire4l  forbotli  tension  and 
uoinpreHMion  and  add  %  of  the  Icsxer  area  to  tlie  ^Mfaler. 

For  combined  dead,  live,  and  wind  load  stresses  strani  30  per  cent 
hiKher  ibaii  tor  dea<i  and  live  load  only. 

The  effect  of  reversion  i  f  stresses  in  case  of  wind  loatls  is  to  lie 
ignored. 

No  impact  is  to  be  added  for  centrifugal  and  traction  loads. 


370 


DB   I'ONTIHUS. 


> 

< 


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TOTAL  END  SHEAR  l^ 


rOTAL  END  SHEAR  IN  POUNDb. 


P  L  AT  LCr. 


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>•>     .- 


EQUIVALENT  UNIFORM  LOA 

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EQUIVALENT  UNIFORM  LOA 


LENT  UNIFORM  LOAD 

e.         Si 


IN   POUNDS. 


PLATE  III. 


LENT  UNIFORM  LOAD  IN   POUNDS. 


100         120  14<l  lU) 


SPAN  IN  FEE 
'AK)  221)  240  200  280  300 


'       100         1^0  110  100  180  200  ;i30  240  200  280         300         3 

SPAN  IN  FEE 


SPAN  IN  FEET.  PLATE rv. 

260  2«U         30U         3iJ0  340  300  380         400  420         440  460  480        000 


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SPAN  IN  FEET. 


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SPAN  IN  FE! 

PLATE  Y. 


3()0  350  KX) 

SPAN  IN  FEET, 


100  1^  14D  160 

LENGTH  OF  SPAN  IN 


PLATE  VI. 


)AD  FOR  ELECTRIC  CARS  FOR  HIGHWAY  BRIDGES. 

J.A.L.  Waddell, 
Consulting  Engineer, 

Kansas  City,  Mo. 


n 


4ari 


rtrr 


A 


m 


1,800  z 

1,600  < 

O 

J 
1,400 


1,200 
1,000 

800 

600 

400^ 


mt!fs^mii.y 


li»  140  160  180 

LENGTH  OF  SPAN  IN  FEET. 


m 


300 


WIND  LOAD  IN  POUNDS. 


0) 

Tl 
> 


m 
m 

H 


WIND  LOAD  IN  POUNDS, 


PLATE  VII. 


>000 


WIND  LOADSFOR   HIGHWA'^ 

J.A.L.  Waddell, 
Consulting  Engineer, 

Kamsas  Citv,  Mo. 


SPAN  iN  FFE7 


PLATE  VIII. 


.DSFOR   HIGHWAY  BRIDGES. 


J.A.L.  Waddell, 
Consulting  Engineer, 

Kansas  Citv,  Mo. 


1000 


SPAN  IN  FFET. 


im 


30%  m  'M 

•     ONE  ARM  OF  DRAW 


PLATE  IX. 


DIAGRAM  OF  REACTIONS 

FOR 

BALANCED  LOADS 

ON 

DRAW  SPANS. 

COMPUTED  BY 

J.A.L.  Waddell, 
Consulting  Engineeii, 

Kansas  City,  Mo. 


-5 


short  mid. 


40ft  <M  <)(¥ 

ONE  ARM  OF  DRAW  SPAN. 


2     3     4     6      0      7      8 

NO.  OF  PANEL  FROM  END  OF  ANCHOH  ARM. 


9 


10 


It 


pi:atex. 


1  I      \-H+<    ! 


XI 


;i!tit: 


(ITO 


CURVESHOWINGWEIGHTS  OF  TRUSSES  OF  CANTILEVER 
AND  ANCHOR  ARMS  OF  CANTILEVER  BRIDGES  IN  PER- 
CENTAGES OF  AVERAGE  TRUSS  WEIGHT  FOR  ONE  PANEL 
OF  SUSPENDED  SPAN. 


i.|U-;.i: 


rrrrrnT 


550 


M}±>\ 


^  ■  .  .  I-: 


?TrM:;;-f 


li 


Hi 


:!;m 


450 


400  u. 


•m 


350o 


11 


Tr;-, 


300 


350 


ttC! 


\t<.'.:: 


200 


:  m 


^-m 


;:;t. 


1^ 


mi': 


150 


il 


ii. 


Hi 


100 


For  weight  of  tower  panel,  multiply  the  percentage  given  for  panel 
'  anchor  arm  next  to  tower  by  1.8. 

For  Span3  with  Subdivided  Panels,  a  main  or  double  panel  is  to 
}  used  as  the  basis  of  Calculation. 

Metal  in  A  no  borages = 5  )<  of  total  weight  of  entire  superstructure. 

Metal  on  Main  Pier8=|-  of  weight  of  metal  in  Anchorages. 

The  weights  of  Lateral  Sy'>  ems  are  determined  in  the  same  manner 
3  the  truss  weights. 


50^ 


10 


10  98765432 

NO.  OF  PANEL  FROM  END  OF  CANTILEVER  ARM. 


INDEX. 


PAGE 

"  A  "  truss-bridges,  advantages  of 0 

'■  A  "  truss-bridges,  history  and  description  of 3 

Abandoned  drawings 342 

Abuse  of  man-power  nuulunery. , 128 

Abutting  ends 174 

Abutting  ends,  workmanship  or 257 

Accessibility  to  paint-brusli 25 

Accidents,  responsibility  for 203 

Accuracy  in  base-line  measurements 317,  318 

Accuracy  in  punching 254,  287 

Adherence  to  principles  in  designing 4 

Adjustable  members 149, 171 

Adjustable  rods  for  draw-spans 102 

Adjustment  of  draw-spans 128 

Adjustment  of  rollers  for  draw-spans 19() 

Adoption  of  subpunchiiig  and  reaming 9 

Advantages  and  disHdvaiiliiges  of  high  and  low  bridges..    119 

Advantages  of  combined  bridges 133 

Advantages  of  lift-bridges 113 

Advantages  of  medium  steel 8 

Advantages  of  soft  steel A 

Advantages  of  the  arch < .» 

/Esthetic  reform  in  bridge-building 45 

^Ksthetic  reform  in  bridge-building,  opposition  to 45 

/Esthetics  in  cantilever  bridges 61,  (52 

.Esthetics  in  design 39 

.Esthetics  in  design,  fundamental  princijile  of 47 

iEsthctics  in  East  Omaha  draw-span 48 

(Esthetics  in  general  engineering  constructions 42 

Esthetics  in  painting 52,  53 

/Esthetics  in  j)ier  designing 53,  54 

Alteration  of  drawings 243, 263,  342 

Ambiguity  in  stresses  in  ordinary  draw-spans 121 

Ambiguity  in  trusses 219 

373 


zu 


INDEX. 


PAnE 

American  Institute  of  Arcliitoets 284 

Analytical  nictliud  of  liiuiin^  stresses 3.'{;; 

Ancliora;j;('  a<j;ainst  wind-pn'ssurc  in  cantilever  bridges.  .     (j(i 

Andioraf^e  details  ft»r  cantilever  hri<lges (i(i 

Anchoraf^e  for  lower  tracks  of  drums !!);'< 

Anchora^!  for  railroad  bridges 150 

Anchorages  of  columns  to  pedestals 27 

Ancliorages  for  draw-spaiis 124,  IDS 

Anchorages  for  elevated  railroads 9J 

Anchorages  for  trestles,  \v«'ights  of 90 

Anchorages  of  cantilever  bridges,  concrete  for 0(1 

Anchorages  of  cantilever  bridges,  weight  of  metal  in..  . .     7;$ 

Anchorages  of  cantilever  bridges,  wells  in 6«> 

Anchor-arms,  economic  lengtlis  of 7P 

Anchor-arms  of  cantilever  iu'idges,  stresses  in 5^ 

Anchor-bars  for  cantilever  bridges 6(i 

Anchor-bolt  connections  to  columns 181 

Anchor-bolts,  specifications  for 259 

Anclioring  spans  to  bearings 24 

Anchor-piers  of  cantilever  bridges.  w«'iglit  of  masonry  in.  (i7 
Anchor-spans  of  cantilever  bridges,  proper  lengths  for..  .  75 
Anchor-spans  of  cantilever  bridges,  weight  of  metal  in, 

73,74 

Angle  measurements 323 

Angles  connected  by  one  leg  only,  proportioning  of 170 

Angles  connected  by  one  leg  only,  tests  of 27 

Annealing  255 

AnneaJing  test-specimens 247 

Appearance  of  curvature 20 

Appearance  of  structure  to  be  governed  by  location. ...     46 

Approaches,  flooring  on 210 

Approaches,  ornamentation  of 53 

Appropriate  ornamentation  for  bridges 47 

Arches   79 

Arches,  cantilevered 80 

Arches,  curvature  of 84 

Arch   depi  hs 84 

Arches  for  higlnvay  bridges 81 

Arches,  future  investigations  concerning 85 

Arches,  hinges  for 84 

Arches,  spacing  of .  . .  . ." 85 

Architects'   charge^i  against   American   bridge-designers 

for  want  of  taste 40 

Architectural  effect,  provision  for 17 

Arkansas  River  Bridge 310 

Arrangement  of  members  in  pairs 21 


INDEX. 


375 


PAOB 

Asspnihlinp  of  turntahlos  in  shops z^Hi 

AHS(»<iatioii  of  iiiM|)ccl(irs.  |)|'()|)ohc(1 '1H',\.  2H7 

.Vssiimcd  uplift   for  liifjiiway  (lni\v-H|mnH 2'M 

Assiniicd  uplift  loads  for  draw-spans |H4 

AssuuH'd  ii|)\\ard  dcid  load  read  ion  in  draw  spans 121' 

Assuiucd  iipuanl  u  lution  niethod 270 

AtfacliintMit  of  mlunin  tVct 148 

Attacliuu'Mt  of  -uspcudj'd  spnhs  lo  cant  ilever-anns (55 

Author's  iustrurtions  to  tit'ld-cnjifiuccrs 28H 

Autiior's  stanchird  insli  uctions  for  dieoking  drawings..  840 

Author's  stauihird  instructions  to  nictal-inspcftors 284 

Au.\iliary  triangul<tti(ui-huhs 324 

Avoidance  of  skcwdaidgcs 18 

Avoidance  of  torsion 20 

Axiom  in  arciiitccturc.  fundamental 42 

Had  inspection 281 

balanced  loads  f(n'  draw-spans 184 

l?allast  tanks  for  Ilaisted  Street  Lift-Hridge 110 

liallot   on   The  Coni[»romi.se   Standard    System   of   Live 

Loads,  etc 207 

Bascule   bridges 105 

IJasedine   measureii.i  n!s   317 

Hase-lines,  lengths  of 322 

Base-lines,  location  of 321,  322 

Batten -plates   174,  232 

Batter  for  piers 305 

Batter  for  trestle  colunnis 87 

Bearing-plates  for  plate-girder  spans 169 

Bearing-plates  over  drums 200 

Bearings  for  arches 289 

Bearings  upon  masonry 150 

Beautifying  bridge-designs  without  increase  of  cost 40 

Beauty  of  j)roportion  and  mat  hematics 44 

Bed-plates 177 

Hed-rock,  ])reparation  of,  to  receive  masonry 291 

Bending  due  to  weight  of  member 159 

Ben<ling  moments  on  ])ins 157 

Bending  moments  on  top  chords 158 

Mending  on  inclined  end  posts 159 

I'.ending  on  inclined  end  pt)sts  of  highway  bridges 220 

Bending  testa 248 

Bent  eyes 178 

Benzine  for  wRshing  drawings 340 

Bessemer  steel 245 

Best  kinds  of  metal  paint 10 

Best  lengths  for  tape  lines 318 


370 


INrvRX. 


PAfiE 

lieat  method  of  letting  bridges 3 

Best  sections  for  columns fi'.> 

Best  span-lengths  for  bridges ;}2!l 

Best  span-lengths  for  trestles 8G,  180 

Bevel  gears .   2i)\ 

Binding  calculations 345 

Binding  drawings 344 

Blunders  in  shopwork 282 

Bolt-holes  in  timber  trestles 27!) 

Bolts  in  timber  trestles 27!' 

Bottom-chord  packing 17<i 

Bottom  ciiords  for  truss  draw-spans 1!)  I 

Bottom  chords,  reversion  of  stresses  in 3J 

Boxes  for  sliafting 20(' 

Braced  steel  piers 31:; 

Braced  towers  for  elevated  railroads !):: 

Bracing  between  cylinders 313 

Bracing  for  draw-spans ]J)| 

Bracing  for  trestle-towers 89,  17!i 

Bracing- fran's  for  deck-sj)ans 107 

Bracing-frames  in  multiple-track  structures 25,  2(> 

Brackets  for  pinions 193 

Brackets  in  elevated  railroads 180 

Brick  piers 30!i 

Bridge  approaches,  ornamentation  of 5."> 

Bridge-designing  stiil  in  process  of  development 43.  44 

Bridge  disasters 131 

Bridges     psigned  by  nianufacturers 3 

Brown  paper  for  drawings 34(5 

Muckle-piat«'  lloors 143 

I'uilt  members,  workmanship  on 25(! 

Builr  stringers  for  highwav  bridges 230 

Burnt  rivets .' 289 

Cables  for  Halsted  Street  Lift- Bridge Ill 

Caissons 292 

(^lissons  of  steel  and  concrete 309 

Caissons  of  tind)er  and  concrete 308 

Calculations   332 

Calculations,  binding  of 345 

Calculations,  filing  of 344,  345 

Calciilations,  graphical 3.34 

Camber  for  draw-s])ans 212 

Camber  for  highway  bridges 220 

Ciunbcr  for  railroad  bridcres 149 

Canal  Street  Fold 


Cantilever-arms, 


Bridge,  Chicago 107 

stresses  in 68 


m 


INDEX. 


sn 


PAOE 


Cantilever  bridfjea 55 

Cantilever  bridges,  a-stlu'tics  in (>1,  «i2 

Cantilever  bridf^es,  andiorage  against  wind-pressure.  . .  .      (Hi 

Cantilever  bridires.  anchorage  details  for (Ui 

Cantilever  biidgcs.  auclior-biu-s  for 00 

Cantilever  bridgfs,  combinations  of  stresses  in GO 

Cantilever  bridge  designing,  system i/ation  of 57 

Cantilever  bridges,  economic  relations  of 57 

Cantilever  bridges,  erection  stresses  in 61 

Cantilever  bridges,  t»xpansion  and  contraction  in 07 

Cantilever  bridges,  linding  stresses  in (il 

Cantilever  bridges,  imi)act  for 01 

Cantilever  bridges,  live  loads  for (iO,  01 

Cantilever  bridges,  pedestals  for (Ui 

Cantilever  bridges,  stresses  in 58 

("antilever  bridges,  widening  of,  over  main  jtiers 02 

Cantilevered  arclies : 80 

Cantilevering  simple  spans  during  ere(!tion 70 

Cantilevers  for  roof-trusses 57 

Care  of  machinery  for  drawbridges 12!) 

Cast  iron 245 

Cast  iron,  specifications  for 252 

Cast-iron  trimmings  for  decoration 52 

Cs'.st  steel,  specifications  for 25)^ 

Cenient  an<l  concrete  testing 2!)2.  2U5 

('entre-l)earing  turntables 12.'5 

Centre  castings  for  draw-s|)ans l!Ki,  107 

Centie  \ni\vs  for  l)ascule  l»ridges lOti 

Centrifugal   load 154 

Chalk,    powdered ;{40 

Changes  in  drawings 24.'{,  .'U2 

Changes  of  teini)erature 155 

Changing  spe<'ilications 2(i4 

Character  of  llang*^  section  of  jtlate-gird*  rs 107 

Charges  by  architects  against   Ameri<'an   bridge-design- 
ers for  want  of  taste 40 

Cheap  iiighway  bridges 2.S0 

(  heap   trestles 8!) 

Che<'ks  on  accr.racy  in  triangulaf  ion :}24 

(hecks  on  correctness  of  percentage   curves   for   canti- 
lever bridges 77,  78 

Checking  calculations .'i.'U,  .3;]5 

('becking  dead   loatl 'MA 

('becking  drawings '.i'.i~,  .'UO 

Cheeking  finishecl  design,  inethotl  for 2!> 

Cheeking  preliminary  drawings 24.'J 


378 


INDEX. 


PAGE 

Chocking  shipping  weights 286 

(  lu'cking  sh()i)-<lia\vings 343 

Choice  of  coiors  in  painting  bridges 53 

( 'hold   ])acking 17U 

Circuhu'.s  to  railroad  engineers 2(5U 

Classes  of  eonibined  bridges 134 

Classifioution  of  higliway  l)ridges 213 

Classification  of  j)iers 304 

Cleaning  metal-work 200 

Clearance  for  elevated  railroads 102 

Clearance  for   liighway   bridges 217 

Clearance  in  ])acking.  provision  for 24 

Clearances  in  railroad  bridges 144 

Ck'aranccs  over   waterw  ays 328 

Clearing  away  rubbisli,  etc.,  for  (rcstles 270 

Closed  tioors  for  elevated  railroads 02 

Closing   thoroughfares..  .'. 2(52 

Coelticient  of  expansion 318.  31!) 

Cofler-danis 301,  302 

Coincidence  of  stress  and  gravity  lines 21 

Collapsing  bucket,  use  of 203 

Collapsing  of  steel  cylinders  in  sinking 291 

Collars  for  siiafting 205 

Colors  for   paints 200 

Column  feet,  elevation  of 27 

Column  feet,  expansion  for 80,  00 

Column  feet,  filling  of 27 

Colunm  feet .  protecting 201 

Columns  acting  as  l)eanis 20 

Columns,  ',«>st  sections  for 00 

Column'    for  elevated  railroa<ls 17'^ 

Columi  s  for  trestles,  batter  for 8' 

Colunns,  s|)lices  in 00 

f  olmiin  lops  in  trestles  and  elevated  railroads 20 

Cond)ination  Hridge  Co.'s  Sioux  City  IJridge 135 

Combinations  of  loads  for  draw-spans ISli 

Combinations  of  stresses  for  plate  girder  draw-spans   18S,  180 

Combinations  of  stresses  in  cantilever  iiridgcs Ot> 

Combinations  of  stresses  in  highway  briilges 225 

Combinati(ms  of  stresses  in  railroail  bridges 158 

Combinations  of  stresses  in  railroad  trestles 158 

Cond)inations  of  stresses  in  trestle  cohunns 88 

Combinations  of  stress«'s,  recording 334 

Combined   bridges 133 

Comparative  economy  «)f  arches  and  simple  truss  spans..  82 

Comparative  imp<n-tance  of  live  and  dead  loads 7 


INDEX. 


379 


PAGE 

Comparative  weight  of  bridges  let  by  the  pound  and 

Ity   luiii[)  sum 2 

ComiM'tition  in  inspection 282 

Competitive  designs,  evil  ell'ects  of 13!) 

Competitive  plans I 

Complete  data,  neeessity  for 18 

Composition  of  rolled  steel 245 

Comj)ound  web-plates 174 

Compression  flanges  of  plate  girders,  sections  of 167 

Compression    formuhe 1;>(» 

Compromise  Standard  System  of  Live  Loads,  etc 2()i'i 

Con<'eptrated  loads  for  highway  bridges 222,  22:1 

(  oil'  iusion    340 

Concrete  for  anchorages  of  cantilever  bridges (Mi 

Concrete,  injury   tt) 293 

Concrete    mixing 2!)3 

(  oncrete  piers 310 

Concrete  piers,  forms  for 292 

('(mcrete,  testing  materials  for 292 

Connecting-plates  for  riveted  giiders.          170 

Connecting  web-angles  to  chords  by  o.         "  mily 96 

Connection  for  shoes  at  ends  of  draw-spa    -  :*I0 

Connection  of  columns  to  masonry .  .  181 

Connection  of  suspended  spans 69 

Connections  of  tloor-beams  to  posts 148 

Consideration  of  iiuality.  freijuency,  and  probability  of 

stresses 22 

Consultation  concerning  architectural  features oO 

Contem|tt  of  engineers  for  architectural  features  in  de- 
signing    44 

Contents  of  drawing-sheets ,338 

Contents   of  stress-diagrams 336 

(^tntiiuiity  of  stringers  in  railroad  bridges 160 

C«  ntinuous  spans 148 

Co.itraetion,  provision  for 23 

Convergence   of  engineering   ))ractice  and   architectural 

ideals ,47.48 

Correction  for  temj)erature  in  base-line  measurements..  319 

Correctness  of  impact  formula' 7.  8 

Cost  of  traveller  alFected  by  truss  depth 32 

Cdunterbricing  in  highway  bridg«'s 219 

Counters    171 

Counterweighted  basciile  bridges 106 

Counterweights  of   Ilalsted  Street   Lift-Bridge 108 

Couplings  for  shaftings 20o 

Cover-plates  for  plate  girders 167 


380 


INDEX. 


Crimping  of  web  stiffening  angles 94 

Cross-section  paper  for  bridge  calculations 332 

Cross-ties  141 

Crowfoot  seams 297 

Cupped  bearings  for  end  rollers  of  draw-spans 210 

Curvature  in  chords,  ai)])earance  of 20 

Curvature  of  arches 84 

Curvature  of  top  cliord 51 

Curved  members  in  English  bri<lges 20 

(^urved  members,  ol)jecti()ns  t<K 19 

Curve  of  pressure  for  masonry  piers 305,  300 

Curves  of  weights  for  cantilever  bridges 72,  74 

C^utting  oil"  idle  corners 254 

Cutting  off  tops  of  columns  of  elevated  railroads 99 

Cylinder  piers 311 

Cypress    299 

Damages   203 

Dapping  ties 1-12 

Data  for  elevated  railroads 102, 331 

Data  for  layout  of  strticture 330 

Data  for  spans,  list  of 332 

Data,  necessity  for  complete IS 

Dead  load ' 1.12 

Dead  load,  preliminary 333 

Dead-load  reactions  in  draw-spans,  upward  assumed.  . .  .    122 

Dead  loads  for  draw-spans 184 

Dead-load    stresses 335 

Dead-load  stresses  in  di  iw-spana 122 

Decoration,  legitimiite  and  illegitimate 40 

Defective  uork 2(i2 

Defects  in  iimbcr 299 

Deflection  of  draw-spans 212 

Demoralized   inspection 282,  283 

Depth  of  truss,  economic 30 

Depths,  effective 1 4  "> 

Dej)ths  of  arches H  t 

Deptlm  of  drums 123 

Depths  of  longitudinal  girders  for  elevated  railroads.  .  .  .    102 

Depths  of  trusses  for  draw-spans IS:? 

Designing,  errors  in fi 

Designing  of  piers 30 1 

Des  Moines  River  liridge  piers 315 

Detail  drawings 243 

Detail  drawings,  method  of  making 337.  338 

Detailing 138 

Detailing  of  members  having  excessive  strength 23 


1 


INDEX. 


381 


PAQK 

94 
332 
141 
21)7 
210 
20 
84 
51 
.  20 
10 
),  300 
?2,  74 
.  254 
.  00 
.  311 
.  200 
.  2t)3 
.  142 
12,  331 
.  330 
,  .  332 
..   IH 
.  .  ir)2 
.  .  333 
122 
184 
33.; 
122 
.  40 
.  202 
200 
.  212 
2.  283 
.  30 
.  14") 
St 
.  123 
.  102 
.  !  K3 

r> 

.    301 
..   315 
243 
37.338 

.    138 
.  .     23 


PAOK 

Details  of  dpsi^n  for  hi»Th\vay  viaducts 230 

Details  of  dcsififn  for  open- webbed,  riveted  girders 100 

Details  of  design  for  pin  connected  spans 170,  228 

Details  of  design  for  j)late-girder  drasv-spans 188,  100 

Details  of  design  for  plate-girder  spans l<i(i.  228 

Details  of  design  for  rolled  I-beam  spans 100,  227 

Details  of  design  for  trestles  and  elevated  railroads 178 

Details  of  design  for  truss  draw-spans 101 

Details  of  drums  and  turntablef) 102, 241 

Details  of  highway  bridges 2.30 

Details  of  operating  machinery  for  draws 204 

J  )etail8  of  trestles 87 

Details,  tests  of 2.33 

Determination  of  power  for  operating  and  lifting  draws.  202 

l)eveloj)nient  of  faculty  of  judgment 14 

Diagonals  for  lateral  systems 172 

Diagonals  for  vertical  sway-bracing 172 

Diagram  of  reactions  for  draw-spans 141 

Diameters  for  drums 123.  124 

Dimensions  for  trestles 270 

Dimensions  of  drawings 344 

Directions  to  Contractor 202 

Direct  tension,  rivets  in 25 

Disadvantages  of  combined  bridges 133 

Disadvantages  of  the  arch 70 

Disagreements  bet'v°en  manufacturers  and  engineers.    10,  11 
Distance    between    expansion    points    in    elevated    rail- 
roads     95, 180 

Distribution  of  load  over  drum 12.3 

Dividing  up  bracing  in  trestle  towers 88,  140 

Division  of  dead  load 152 

Doric  order 43 

Doul>le  concentrated  load  method 200 

Double  rotating  cantilever  draws 103 

Drainage  of  ])ivot  piers 128.  105 

Drawings,  tiling  of .344 

Drawings  for  trestles 277 

Draws,  double  rotating  cantilever 103 

Drift-bolting 270 

Driving  (ield-rivets 24 

Driving  piles 278 

Driving  piles  into  cylinders 313 

Drawbridges  for  various  span  lengths,  styles  of 182 

Drawers  for  tiling  drawings 344 

Drawings 243 

Drawings,  dimensions  of 344 


982 


INDKX. 


PAnE 

Drawin^'-sliet'ts,  contoiits  of ;W8 

iJrawing.s,  making  of '.VM} 

Oia\v-a|»ans,  adjuHlnient  of 128 

Draws,  jniU-hack 104 

Drifting    tests IV) 

J)rillings  for  choinical  analysis 24<),  247 

Drums,  details  of 1!)2 

Drums  for  Ilalstcd  Street  Lift-Bridge   112 

Drum    webs 1!);{ 

Duties  of  bridge  specialist .'{ 

Dynamite  in  pier-sinking,  use  of ;{(>;{ 

Ease  in  designing 1;; 

Kast  Omalia  Bridge i;55,  K{(i 

Kast  Omaha  Bridge  piers 3(13,  .'{J 1 

Eiist  Omaha  draw-span,  equalizers  for 12r> 

East  Omaha  draw-span,  rising  of  ends 12(i,  127 

Eeeentric  loading  from  sidewalks 222 

Eeonomieal  conditions  for  cantilever  bridges r)(i 

Economic  depths  for  triisses  with  polygonal  top  chords..     32 

Economic  depths  of  plate  girders 33 

i.'conomic  depths  of  trusses 30 

Ecv>nomic  functions  of  cantilever  bridges 70 

Economic  layouts  for  viaducts 87 

Ecoi.omic  length  of  anchor-arm  for  fixed  distances  be- 
tween main  piers 7") 

Economic  length  of  main  s])an  in  cantilever  bridges  for 

fixed  distances  between  anchorages 74 

Economic  length  of  suspended  span 70 

Econonnc  panel  lengths 3.3 

Economic  princi|)lc  for  any  layout  of  spans 30 

Economic  rehitions  of  cantilever  bridges 57 

Economics  for  crossings  involving  danger  of  washouts.  .     38 

Economic  span  lengths 33,  34 

Economic  span  lengths  for  elevated  railroads 02,  0.3 

Economic  span  lengths  for  trestles 80,  180 

Economic  span  lengths,  mathematical  demonstration  of.     34 
Economy,    comparative,    for    arches    and    simple    tru.ss 

spans   82 

Economy  in  design 30 

Economy  in  roof-trusses r>7 

Economy  in  trestles  and  elevated  railroads 30,  84 

Economy  in  trusses  with  parallel  chords .31 

Economy,  necessity  for IT) 

Effective  depths Hf) 

EflFeetive  diameter  '.f  rivet lOf) 

Effective  lengt  s . .    145 


Er 
Er 
Er 


ces 


I'AOE 

. . .  :t:is 
...  ;wr) 

. . .    128 

.    .    104 

.  .     24') 

24(».  247 

.  .  .  .    l!>2 

.  ...    112 

.  .  .  .    VM 

:{ 

;$():{ 

. . . .     i:! 

i;i5. 131) 

303,  :{ii 

....  I2r> 

12(1,  127 
....   222 

5(1 

32 
3:5 
30 
70 
87 

7"» 


hords.. 


be- 


lies for 

.    74 

70 

33 

3(1 

r)7 

liouts.  .     38 

33,34 

1)2,  03 

.  . .  .  8(1,  180 
tion  of.  34 
e    truss 

82 

30 

57 

30, 84 

....      31 
....      15 

145 

....    105 
..    ..    145 


INDEX. 


883 


PAGE 

KfToc'tive  .strength  of  plate-girder  webs  in  bending 109 

Ktteet  of  impact,  provision  for 10, 17 

Ktl'eet  of  weight  of  traveller  on  erection  stresses 00 

Kffects  of  changes  of  temperature 155 

Elastic  limits 247 

Electric  lights  for  ornamentation 52 

Electric  motor.",  for  draw-spans 201 

Electric  railway  loads 223 

Electric  railways  on  highway  bridges 133 

Elementary  ])rinciplos  in  outlining  spans 50,  51 

Elevated  railroads 91 

Elevated  railroa<l8,  economy  in 30 

Elevated  railroads,  ornamentation  of 52 

Elevation  of  colunm  feet 27 

Elongaf  ion    247 

End-bearing  rollers  for  diiiw-s])ans 209 

End  floor-beams  for  highway  bridges 218 

End-lifting  apparatus  for  draw-s[)ans 207,  211 

End  posts,  bending  on.    159 

Ends  of  channels,  trimming 170 

Engineer   204 

Engineering    practice    and    architectural    ideals,    con- 
vergence of 47,  48 

English  bridges,  curved  members  in 20 

Equali/AM-s  for  draw-spans 125, 20l 

Equivalent  live  loads  for  elevated  railroads 92 

Equivalent  live  loads  for  railroad  bridges 151 

Equivalent  twisting  moment  on  shafting 200 

Equivalent  uniform  load  method 205,  208 

Equivalent  uniformly   distributed   live  loads  for  high- 
way  bridges 223 

Erasures  on  tracings 343,  344 

Erection 201 

Erection  of  arches 79,  80 

Erection  stresses  in  cantilever  bridges 01 

Errors  for  graphics,  limits  of 335 

Errors  in  designing 5,  12 

Errors  in  triangles 323 

Errors  in  triangulation 317 

f'.stablishment  of  first  principles 12 

Establishment  of  formuloe  for  impact 7, 8 

Estimating  weights  of  details,  liberal  allowance  for 28 

European  practice  in  oesthetic  designing 54 

Excavation  , 291 

Excessive  expansion  joints  in  elevated  railroads 98  • 

Excessive  strength,  detailing  of  members  having 23 


384 


INDEX. 


PAOB 

Expansion  and  contraction  in  cantilever  bridges ()7 

Expansion  and  contraction,  provision  for 2.'} 

Expansion  for  cohiinn  feet  in  trestle-towers 39,90,  149 

Expansion  for  railroad  bridges 149 

J^xpansion  joints 95 

Expansion  pockets 181 

Expansion  rollers 177 

Experiments  on  hinged  columns 14 

Experiments  on  impact 6 

Explanation  of  specifications 5 

Exterior  sidewalks 222 

Extension  plates 170 

Extra  loads  due  to  curvature 151 

Extreme  fibre-stress  for  timber 15(i 

l-'iXtreme  fibre-stress  on  shafting 20*5 

Eye-bars,  heads  of 178 

Eye-bars,  specifications  for 257 

Factor  of  ignorance 7 

Faculty  of  rapid  Judgment 14 

Failures  of  highway  bridges IHO 

Fatigue  of  metals 2;i 

Faults  in  exiatinj^  .'."vated  railroads 95 

Faulty  designing  of  top  chords  of  open- webbed  girders..  97 

Faulty  detailing 138 

Faulty  intersection  of  gravity  lines  in  girders  of  ele- 
vated railroads 96 

Field-riveting  in  riveter*  girders 169 

Field-riveting,  reduction  to  a  minimum 24 

Field-rivets,  ease  in  dri .  ing 24 

Field-rivets,  intensities  for ■ 156 

Field-splicing  for  plalc-girder  draw-spans 189 

Filing  drawings  and  other  data 344,  345 

Filing-tubes 345 

Filling  caissons 292 

Filling  of  column  feet 27 

Filling  recesses  in  metal- work  before  painting 261 

Finding  stresses  in  cantilever  bridges 61 

First  principles,  establishment  of 12 

First  principles  of  designing 12 

Firth  of  Forth  Bridge 67 

Fixed  ends  for  columns  of  elevated  railroads 180 

Flange  splices  in  plate  girders 167 

Flats  for  diagonals  for  riveted  girders 170 

Floating  draws 118 

Floating  timber 299 

Flooring  on  approaches 216 


(; 

(3 
(i 
(J( 
(Jc 

(U 
r.€ 
(U 
iU 

(id 

(!( 
(k 

G( 

Gi 
Gi 
Gf 
Gil 
Gr 


PAO« 

1>7 

2:i 
«.K),  149 
.    UU 
.     95 
.   181 
.   177 
.     14 
6 
5 
.  222 
.   17« 
.   151 
.   15t'. 
,  2(»«) 
.    178 
.  257 
7 
..      14 
..    I'M) 
. .    2:1 
..     95 
i..     97 
..    138 
le- 

..     96 
..   1«9 
..     24 
..     24 
..   15«) 
...    189 
}44, 345 
...   345 
...   292 
...     27 
...  261 
...     61 
...     12 
...     12 
...     67 
...   180 
...   167 
...    170 
...   118 
...  299 
...  216 


INDEX. 


385 


PAQB 

Floor-planks  for  highway  bridges 214 

Floors  for  elevated  railroads 92 

Floors  for  wooden  trestles. 275 

Folding  bridges 107 

Foresights  325 

Forked  ends 176, 233 

Forms  for  concrete  piers 292 

Forms  of  trusses  for  highway  bridges 218 

Forms  of  trusses  for  railroad  bridges 145,  140 

Formulae  for  truss  weights 32!),  330 

Formulw  for  wooden  compression  members 27(i 

Formula  for  impact  for  railroad  structures 7 

Foundations,  testing  of 291 

Fracture 249 

Framed  trestles 273,  275 

Framing  timber  trestles 278 

Frequency  of  stress  application 22 

Full  lengths  of  sections 174 

Full-sized  eve-bars,  tests  of 250 

Full-sized  members,  testing  of 288 

Full-sized  members,  tests  of 253 

Fundamental  axiom  in  architecture 42 

Fundamental  printiiple  of  aesthetics  in  design 47 

Future  investigations  concerning  arches- 85 

CJas-engines  for  operating  draw-spans 120 

Uasliglits  for  ornamentation 52 

(lasoline  engines  for  operating  draw-spans 120, 201 

(Jear  wheels 204 

General  economic  principle  for  all  structures,  involving 

capitalized  cost  *of  deterioration  and  repairs 38 

General  instructions  to  fleld-inspectors 295 

General  layout  of  structure 330 

(ieneral  limits  in  designing  highway  bridges 22() 

(leneral  limits  in  railway-bridge  designing 160 

General  notes  on  detail  drawings 342 

General  principles  for  jiroportioning  details 28 

(Jeneral  principles  in  designing  all  structures 161 

General  provisions  on  methoils  of  testing 246 

(ileneral  specifications  for  liighway  bridges  and  viaducts.  213 
General  Specifications  for  Highway  Bridges  of  Iron  and 

Steel .' 130 

Girders  in  trestles,  spacing  for 87 

Girders  over  drums 199 

Graphical  calculations 334 

Gravity  axes,  intersection  of 20 

grillages 303, 309 


386 


iNDrx. 


PAQK 

Guard-timbers  142 

Guide-chairs   for   rail-lifts 209 

Guidinj;  caissons  and  cylinders  in  sinking 21)2 

Ilulf-tlirough  platc-fjinlcr  spans KiS 

llalsted  Street  Lift-Iiridgo  at  Chicago 108 

Hand-operating   niacliiiicry 200 

Hand-operating  machinery,  necessity  for 202 

Hand-rails  for  highway  bridges 215,  21(» 

Hand-rails,  ornamental 52 

Hand  turning-niacliinery,  fornmhp  for 20;} 

Harlem  River  bascule  bridge  at  New  York 107 

Harshness  of  outlines  of  triiss-bridges 42 

Heads  of  eve-bars ITS 

High  steel,*  use  of 8,  141, 182, 244 

Highway-bridge  failures 130,  131 

Highway  bridges 130 

Highway-bridge  lettings 2 

Highway  bridges,  classification  of 213 

Hinged  ends  for  columns 00,  179 

Hinges  for  arches 84 

Hip  verticals  for  highway  bridges 220 

Hip  verticals  for  railroad  bridges 148 

Holes  in  timber  for  trestles 271) 

Horizontal    bendinp    on   cross-girders   of   elevated   rail- 
roads         98 

Horizontal  sway-bracing  in  trestle-towers 89 

Houses  for  machinery  of  draw-spans 211 

Howe's  Treatise  on  Ardies 81,  82 

Hubs,  Irianguhition 320 

Hydraulic  buflers  for  Hui.ted  Street  Lift- Bridge 109 

Ice-breaks  for  ])iers 308 

Identification  of  metal 240 

Ignoring  bending  in  columns  of  elevated  railroads 100 

Imaginary  superiority  of  cantilever  bridges 55 

Impact 0 

Impact  allowance  for  highway  loads 224 

Impact  allowance  for  railroad  loads 152, 223 

Lupact  for  cantilever  bridges Gl 

Impact  for  highway  bridges 8, 132 

Impact  method,  importance  of 7 

Impact  stresses,  method  of  finding 333 

Importance  of  chapter  on  First  Principles 12 

Importance  of  imjjact  method 7 

Importance  of  rigidity 15,  IG 

Importance  of  scientific  detailing 139 

Improvement  in  highway-bridge  building 131 


LNDKX. 


38; 


Tniprovonicnt  in  insj)Pftion 

Jnipioveim-iitH  in  desij^n  for  llalstcd  Street  Lift-Hridge. 

Inni|)iuity  <>f  lacing  to  nirry  tninsversc  load 

Inclined  end  ponts.  bending  on 

Inclined  end  posts  of  lii;,'h\vay  bridf^es.  bending  on 

Inclined  end  posts,  sectit)n8  of 

Inclining  tniss-planes  of  cantilever  bridges 

Incorrect  assumptions  in  economic  investigations 

Indexing   

Ind»'x   of  tulies 

India   inks 

I  ndices  

I  iidin'ct   wind  load 

Indirect   \vin<l  load  for  dra\v-si)ans 

Inetl'ective  anchorages  for  ehsvated  railroads 

Ingt.'ta    


Inherent  sense  of  fitness 

Injury  to  cimcrete 

Inspecting   rails 

Inspection   

Inspection  of  masonry 

hispection  of  materials  and  workmanship 

Inspection  t)f  painting 

lnsi)ection  of  paints 

Inspection  of  stone  for  masonry 

Inspection  of  timber 

Inspection,  strictness  of 

Inspectors'  reports 

Instructions   for  c-iiecking   drawings 

Instructions  to   field-engineers 

Instructions  to  metal-inspectors 

Instruments   fr)r  triangulation 

fnsulticieucy  of  rivets  in  elevated  railroa<ls 

Insullicient  bases  for  pedestals  of  elevated  railroads.  .  .  . 
Insullici«'nt     bracing     betw((en     adjacent     longitudinal 

girders  in  elevated  railroads 

Insullicient  bracing  on  curves  in  elevated  railroads 

Intensities  of  working-stresses 

Intersection  of  gravity  axes 

Intricacy  of  the  science  of  bridge-designing 

Intrusting  designing  to  experts 

Inveatigati(ms  concerning  paints 

Iron  rods  for  base-line  measurement 

1  struts,  spacing  of  angles  in 

Jack-knife  bridges 

Jaw-plates 


I'AOK 

113 

2() 

l.-)!) 

170 
t)-i 

:{() 
:iA.-> 

i:i4 

IH;-) 

100 

245 

14 

•>m 

290 
244 
294 

281 
290 
2G0 
29»i 
297 
203 
28() 
340 
28S 
284 
323 
!)(> 
101 

97 

97 

155 

20 

15 

5 

9 

317 

177 

107 

i'33 


388 


INDEX. 


PAGR 

JofTpiaoii  City  Bridge,  anchorage  for  draw-spi'Ti 124 

•leffer«oii  City  Ikidge  piers 313 

Jefferson  City  Uridge  triangulation 324 

.JoiatH  for  highway   bridges 214 

Judgment  in  designing,  necessity  for 15, 23 

Kansas  City  &  Athinti(?  Railway  l>ridge 114.  135 

K.  C.,  1*.  &  (J.  Railway  bridge  over  tlie  Arkansas  River..   310 

Keyiioies  for  hand-levers  of  draw-spans 211 

Knots  in  timber 2U9 

Labor-saving  devices,  use  of 13 

Lacing  174,  175, 232 

Lacing,  incapacity  to  carry  transverse  load 20 

Lack    of   0,'sthetic    treatment    in    American    bridge   de- 

.    signs,  rea.sons  for 39 

Ijack  of  economy  in  cantilever  bridges 55 

l^ack  of  rigidity  in  cantilever  bridges 55,  57 

Lack  of  rigidity  in  suspension  bridges 57 

Ijansdowne  cantihfver  bridge 07, 08 

Latches  for  draw-spans 209 

Lateral  bracing  for  highway  bridges 219 

Lateral  bracing  on  curves  in  elevated  railroads 180 

Lateral  struts,  sections  «)f 17i 

Lateral  systems  ior  draw-s|)ans 192 

Latenil  systems,  weight  per  foot  of 329 

Lattice  girders,  objection  to 19 

Laying  out  work 328 

I^ayout  of  structure,  general 330 

Layouts  for  elevated  railroads 331 

Layouts  for  viaducts,  economic 87 

Least  thicknesses  of  drums 19-1 

Least  thickness  of  metal  in  highway  bridges 22(5 

Legitimate  and  illegitimate  decoration 4(5 

Length  of  centre  panel  for  draw-spana 183 

Length  of  pin-plates 14 

Lengths,  effective 145 

Lengths  of  base-lines 322 

Lenticular  arches 83 

Lettering 339 

Letting  bridges  by  the  pound 139 

Levelling  off  tops  of  pivot  piers 195 

Levels  in  pier-sinking 320 

Liberal  allowance  in  estimating  weights  for  details 28 

Lift-bridges 108 

Lifting  deck 114 

Tamitation  of  scope  of  all  specifications 1,') 

;.i;niting  lengths  of  caj^tilever  b"M'ketf». ....,,..,...,..  %W 


IND£X. 


389 


,  31) 
.  55 
55,  57 
.  57 
07,  «B 
.  209 
.  219 
.  180 
.  171 
,  192 
.  329 
19 
328 
330 
331 
87 
191 
22(5 
4() 
183 
14 
145 
,  322 
.  83 
.  339 
.  139 
.  195 
.  320 
.  28 
.  108 
.  114 
.  15 

.  m 


i 


PAOI 

Limiting  lengths  of  continuous  steel  stringers 227 

Limit  of  \vorking-Htre88 8 

Jjimits  in  bridge-designing 160, 220, 227 

Limits  of  crroiH  for  grapliics 335 

l^inkw  for  toggles  of  draw  nuicliinery 208 

Jjist  of  data  tor  spans 332 

jjive  and  dead  loads,  eoinparative  importance  of 7 

Liv<^  loads  for  eantih^ver  bridges 00, 01 

\A\i'  loads  for  eonihiiied  hri<lges 13(1 

Live  loa<ls  for  draw-spans 183 

Live  loads  for  elevated  railroads 91,  151 

Live  loads  for  highway  bridges 221 

Live  loads  for  railroad  bridges,. 151 

Loads  for  draw-spuns 183 

liOads  for  highway  bridges 221 

Loads  for  piles  in  cylinders •.  313 

Loads  for  railroad  bridges 15'> 

Locating  piers  during  sinking 32(> 

Location  of  base-lims 321,  322 

Location  of  structiu'e  as  atl"«>cting  its  decoration 40 

Long   panels 51 

Jjong  sights  in  triaiigulation 323 

Long  steel  taju",  measurements  with 319 

Loop-eyes    178 

] ioose  rivets 255 

Lower-chord  packing 17(5 

Lower  lateral  systems  of  higliway  deck-bridges 219 

Lower  lateral  systems  of  railroad  deck-bridges 219 

Lower  tracks  of  turntables 194 

Lubricating  sliding  and  rolling  surfaces 28!> 

JjUmp-sum  bids 2 

Machinery  for  drawbridges,  care  of 129 

Machinery  for  draw-spans 201 

Macliinery  for  Halsted  Street  Lift-Bridge Ill 

Machinery-houses  for  draw-spans 211 

Main  central  spans  of  cantilever  bridges,  stresses  in. .   59,(50 

Main  nusmbers  of  highway  truss-bridges 218 

Main  members  of  railroad  truss-bridges 140 

Making  detail  drawings,  method  of 337 

Making   drawings 335 

Man-power  macliinery 128 

Man-power    operating    apparatus    for    Halsted    Street 

Lift-Bridge  112 

Man-power  operating  machinery,  necessity  for 202 

Margin  lines  for  drawings 339 

Mask  of  ornamental  cast  iron 42 


BdO 


li<i)EX. 


Masonry,  bearings  upon 156 

Masonry,  inspection  of 294 

Masonry  piers 305 

Materials  for  draw-span^ 182 

Materials  for  liiglnvay  1,  "Mges 21.'{ 

Materials  in  railroad  sti  actures 141 

Materials,  specifications  for 24."} 

Mathematical  demonstration  of  economic  span  length..  .     34 

Mattresses  around  piers 314 

Measurement  of  angles 323 

Measurements  on  the  ice 31S 

Measurements  with  long  steel  tape 31!> 

Measure  of  strength  of  structure 1(> 

Mechanical-power  turning-machinery,  formulaj  for 203 

Medium  steel,  reason  for  using 8 

Memphis  Hridge 08 

Menominee  Canal  bascule  bridge  at  Milwaukee 107 

Metal    244 

Metal- work  for  timber  trestles 270 

Method  of  checking  finished  design 21) 

Method   of   determining    i)0\ver    required    for   operatiiig 

and  lifting  draws 202 

Method  of  making  detail  drawings 337 

Method  of  measuring  with  steel  tape 31!) 

Method  of  utilizing  ecjuivalent  loads 207 

Methods  of  operating  revolving  draw-spans 120 

Methods  of  pier-sinking 301 

Methods  of  study  of  testhetics  in  any  bridge  design..  .    .     4!> 

Methods  of  testing 240 

Metro;  olitan  Klevate<i  bascule  bridge,  (Chicago 100 

^linimum  fees  for  professional  wo»k 28^ 

Minimum  thicknesses  of  drums 104 

Mininuim  thickness  of  metal  in  highway  bridges 220 

Mistakes  of  construction 12 

^Mixing  concrete 203 

Modern  bridge  an  offence  to  the  landscape 41 

Moments  on  pins '. 157 

Movable  bridges  in  general 103 

Multijde  systems  of  cancellation 18 

Multiple-track  structures,  bracing  frames  in 25,  20 

Name-plates   ];")() 

Name-plates,  specifications  for 25!) 

Necessity  for  allowance  for  variation  in  pier-sinking...   310 

Necessity  for  bridge  specialist 3 

Necessity  for  complete  data ]S 

Necessity  for  e.vperiments  on  im])act 17 


tiTDEX. 


tn 


202 
:VM 

2(i7 
.  120 
.  301 
.  4!) 
.  240 
.  100 
.  284 
.  194 
.  22(> 
.      12 

.  2ya 

.     41 

.  ir)7 

.    103 

.      18 

25,  2() 

,  .    !;')() 

.  .    2;")!) 

.  .  31(> 
*> 
<i 

..      18 

. .     17 


TAQE 

.Veoossity  for  siiljpunfhing  and  ronming f/ 

Necessity  for  syiiiiiictry  in  rivotin^ 22 

Necessity  for  symmetry  of  section 21 

Necessity  for  thorouf^li  inspection 300 

Net    sect  ion 157 

Net  sections  nciir  pinholes 170 

Niaf,'iiiii  cantilever   bridj^e 314 

Nippon  Railway   hridj^es 02 

Notcliinj,'  ends  of  (  ross-frirdois 173 

Notes  for  trian^nilat  ion-work 323 

Notes  on  drawings 33it 

Nund)er  of  colunnis  for  elevated  railroads 03 

Nundier  of  rivets  at  ends  of  [date  girders 10!) 

Niunher  of  test  pieces 24!( 

Nuts,  specifications  for 25S 

Oaks 298 

Objections  to  cmved  members 10 

Objections  to  iar<;e  bascule  bridges 107 

Objections  to  lattice  ginb'rs 1!) 

Objections  to  redundant  nuMubers 19 

Objections  to  skew-bridges 18 

Objections  to  star-struts 28 

(^bjj'ctions  to  two-rivet  connections 25 

Obj<H-tions  to  \Vhip])le  trusses 19 

Obstacles  ill   |)ier-sinking 302 

OtRce  mati'rials 345 

<)flice  practice 328 

Oil-grooves    194 

Omission   of  diagonals  in   lateral  systems  of  cantilever 

bridges   05 

Omission    of      laoliragm    webs    in   c(dumns   of   elevated 

railroads    100 

Opeii-ditMlging  process 302.  803 

0,,erating  cables  for  Halsted  Street  Lift-Hridgo 112 

Operating  niachineiy  for  Halsted  Street  Lift-Hridge.  . .  .  ill 

Opposition  of  sti  >  I  nianufailurcrs  to  iinprovenicnt 10 

Opjiositioii  to  lestlietic  reform  in  bridge-building 45 

Opposition  to  ]Udposed  metliods  ol  design 4 

Ornamculal  i-asting,  mask  of 42 

Ornamcnial    hand-rails 52 

Oniaiiiciitalioii  for  bridges.  anj)ro])riate 47 

Ornamentation  of  bridge  approaches 53 

Oi  iiamciitat  ion  of  elevated  railroads 52 

Ornament  at  ion  of  ])ortals 52 

Ornamentation  of  viaducts 52 

Outlines  of  truss  bridges,  harshness  in 42 


392 


INDKX. 


PAGE 

Outlining  spans,  study  of  testlietifs  in 50 

Overturning  of  piers,  resistance  to 307 

Paint-brush,  accessibility  to 25 

Painting,  aesthetics  in 52,  53 

I'ainting,  inspection  of 290 

Painting,  investigations  for !) 

Painting,  specifications  for 25iJ 

Pairs,  arrangement  of  members  in 21 

I'altry  brackets  in  elevated  railroads t)9 

Panel  lengths,  economic 33 

l*a])cr  on   Klevated  liailroads 91 

Part  henon   43 

I'aved  doors 21(5 

l'edestal-('a|)s  for  jlevated  railroads 93 

Pedestals   177 

I'ech'stiils  for  cantilever  bridges OtJ 

Pedestals  for  elevated  railroads,  sizes  of 181 

IVdestals  of  t«)\vers  in  llalsteil  Street  Lift-IJridge 109 

Percentage-curves    for    weights    of    cantilev«'r    bridges, 

72,74,77,78 

Petit  truss,  superiority  of 19 

Piece-work    2(54 

Pier  centres,  triangnlati(m  to 325 

J'iers,  designing  of 301 

Pier-sinking   315 

J'ier-sinkiiig,  methods  of 301 

Pile  foundations 303 

Pile   piers 314 

Piles  in  concrete 303 

Piles,  specifications  for 277 

Pile-trestles    273 

Piling  293 

Pin-connected  trusses  for  elevated  railroads 98 

Pines 298 

Pinholes : 1 7(5 

Pinholes,  specifications  for 257 

Pinion  brackets 1 93 

Pinions  for  draw-spans 125, 201,  205 

]'in-metal 251 

Pin-plates   175, 233 

Pin-proportioning  17(J 

Pins,  bending  moments  on 157 

Pins,  specifications  for 258 

Pipes,  removal  of 291 

J'ivot-piers.  drainage  of 128 

Placing  concrete  under  water 283 


IKDEX. 


393 


pagk 

50 

307 

25 

)2,  53 

290 

9 

259 

21 

99 

33 

91 

43 

21(> 

.  93 

.  177 

.  ()G 

.  181 

.  109 

77,  7H 
.  19 
.  2H4 
.  325 
.  301 
.  .  315 
.  .  301 
.  .  303 
.  .  314 
. .  303 
.  .  277 
.  .  273 
. .  293 
.  .  98 
.  .  298 
.  .  170 
..  257 
.  .  193 
01,205 
..  251 
75, 233 
..  170 
..  157 
..  258 
..  291 
..  128 
..  293 


I 


i 


Planinor-arums  19,S 

Plate-jjfirder  driiw-spans,  field-splicing  for 189 

Platf  {girders,  economic  depths  of '.m 

Pneumatic  process 302 

Pouy-trusH  bridges 2 IS 

Pony-trusses  for  elevated  railroads !I8 

Portal  bracing  for  railroad  bridges 147 

Portal  bracing  in  liighway  bridges 21!) 

Portals,  ornamentation  of 52 

Posts,  sections  of 171 

Powdered  dialk  or  talcum 34(» 

Power  for  draw-spans 12(»,  201 

Power  for  op«'rating  .lelVerson  City  draw  span 121 

Power  re(|uired  foi  <ipcrating  and  lifting  draws 202 

I'rejudicial  ene<t  of  competitive  tlesigning 2,  3!> 

Pieliminary  data  for  elevated  railroads 102 

Preliminary  dead  load 333 

Pre|)aratioii  of  Ijcd-rock  to  leceive  masonry 291 

Preservation  of  metal- work 10 

Pressure  on  screw-threads 208 

Prevention  of  rising  ends 209 

Price  for  lirst-class  inspection 288 

Principles  of  designing 12 

Principles  in  designing,  adherence  to 4 

i'robability  of  stress  application 22 

Projecting   web-plates 257 

Projection  of  i)iles  into  cylinders 313 

Proper  colois  for  painting  bridges 53 

Proper  conditions  for  arch  bridges "9 

Proper   distance   between   expansion   j)oints    in    elevated 

railroads    95 

Proper  kinds  of  draws  for  various  crossings 118 

Proper  lengths  for  anchor-spans 75 

Proper  live  loatls  for  cantilever  bridges 00,  01 

Proper  loads  for  piles  in  cylinders 313 

Proper  locations  for  cantilev(>r  bridges 55 

Proper  locations  for  suspension  bridges 57 

Proper  method  of  letting  bridges 3 

Proper  pricj  for  lirst-class  inspeition 28S 

Proper  relations  between  outline  diniensions  of  spans.  .  .      51 

Proportioning  details,  general  principle  for 28 

Projtortioning  of  built  members 174 

Proportioning  of  jdns 17(5 

Proportioning  turntabb's  of  draw-spans 128 

Proportioning  web-anglcs  coniu'cted  by  one  leg  only.  ...    170 
proposed  association  of  inspectors 283 


394 


IKDEX. 


.  PAGE 

Protecting  column  feet 291 

Protecting   hubs 320,  321 

Provision  for  clearance  in  packing 24 

Provisi(»n  for  i>Hect  of  impact ](},  17 

l^iovision  for  expansion  and  contraction 23 

Piovision  for  factor  of  ignorance 7 

Provision  for  thrust  transference 20 

PwU-back  draws 104 

Punching,  accuracy  in 254, 287 

Punching  and  reaming 250 

Quality  of  stress,  consideration  of 22 

tjuarry-l)ed   .  .  . 297 

(Quarry-sap    297 

Quenching 248,  249 

Packs  f«)r  turning  draw-spans 198 

Padial  struts  for  drums 193 

Kail    inspection 290 

IJail  lifts  for  draw-spans 208 

Kailroad  structures,  specifications  for 141 

Kails  for  elevated  railroads 93 

Kaising  column   feet 181 

Raising  colunui  feet  of  elevated  railroads 101 

Katio  ot"  Icngtli  to  least  radius  of  gyration 100 

Katio  of  working  intensity  to  elastic  linnl 8 

Katios  of  length  to  least  radius  of  gyration  in  liighway 

Itridgcs   227 

Katios  of  truss-depth  to  span-length 32,  33 

Katios  of  weiglits  of  cantileviMs  and  lived  spans 75 

Keaiinng    250 

Iveaming,  necessity  for 9 

Keasoiis    for    lack   of   a'stiietic    treatment    in    American 

bridge  designs 39 

Keasons   for   unsatisfactory   conditions   in   existing   ele- 
vated   railroads 24 

Keasons  for  using  medium  steel 8 

Kecordiug  of  cctmbiiuitions  of  stresses 334 

Jiecording  of  wind  stresses 334 

Kecording  progress  of  shop- work 28(i 

Ked  i{ock  Bridge  piers 307 

Ked   Ivock  cantilever  bridge 04,  08 

Keduct  ion  of  area 248 

Reduction  of  lield-riveting  to  a  nunimum 24 

Kedundant   members,  objections  to 1!> 

Ke-entrant    corners 254 

Referencing  hubs 321 

Reform  in  bridge-building,  testhetic 45 


f1 


INDEX. 


395 


PAOK 

21)1 

,:j-21 

'24 
(»,  17 
23 

7 

'20 

104 

'287 

2m 

22 

297 
297 
249 
198 
193 
290 
208 
141 
93 
181 
101 
100 
8 

y 

.  227 
32,33 
.  75 
.  250 
9 
n 
.     39 

.     24 

8 

.   334 

.   334 

.   28() 

.   307 

04,08 

,  .   '248 

, .     24 

, .     19 

. .  '254 

,  .   321 

. .     45 


PAOK 

Reiniorcing-Dlates  at  pinlioloa 14 

Kojectioii  t)t'  innteii  il 202 

lU'latioiis  iK'twccii  priiu-ipal  ilinienHions  of  arches 84 

Relations  iK'twi-eii  various  leading  dimensions  in  canti- 

levoi"   bridges 77 

Removal  of  [)i|)es  and  sewers 291 

Removal  of  surplus  material "291 

Repairing    bridges 139 

Reports  of  tindx-r  inspectors 299 

Re-railing  api)aratus 142 

Resistance  to  overturning  of  [)iers 307 

Responsibility   for  accidents 203 

Resting  longitudinal  girders  on  cross-girders 98,  180 

Reversibility  of  draw-spans 207 

Reversing-stresses 1  ;7 

Reversion  of  sti'csses  in  liottom  chords 32 

Revolving    drawbridges 119 

Rigid  bottom  chords  in  highway  bridges 220 

Rigidity,  importan»-e  of 15,  10 

Rigidity  of  architects'  ideals 42 

Rigid  lateral  bracing  for  railroad  truss-bridges 147 

Rigid   lateral   syst«'ms '24 

Rigid  sway-bracing  for  cantilever  bridges (i5 

Rigid  to])  (  hords  for  draw-spans 183,  191 

Rim-bearing  rnxiis  c«'ntre-bt'aring  turntables 123 

Rising  ends  of  draw-spans 120,  209 

Riv«'t -heads 255 

Hixet-holes    255 

Hiveting 104 

Riveting  in  highway  l)ri(lge8 227 

Riveting,  necessity  for  symnu'try  in 22 

Rivets 255 

Rivets  in  direct  tension 25 

Road   roller 222.  223 

Rolled    steel 245 

Roller  boxes 177 

Roller  plates 177 

Rollers   I77 

Kolleifi  for  draw-spans 15)5 

Rollers  for  draw-spans,  tests  of 253 

Hollers  of  Habted  Street  Lift-Hridge 110 

Hollers,  specifications  for 259 

Koof-trusses    33I 

Roof- trusses,  cantilevers  for 57 

Roof-trusses,  economy  in 57 

Rust-cement   194 


'i^^6 


INDEX. 


PAQB 

Sand-briquette  testa 296 

Scales  for  drawings 338,  342 

Scientitic  detailing,  importance  of 131> 

Screws  for  draw-span  machinery 207, 20S 

Sectional  areas  of  web  members  for  open-webbed,  riveted 

girders  27 

Sections  and  section  lines  (drawings) 341 

Sections  for  columns  of  elevated  railroads })4 

Sections  of  compression  llang(!S  of  plate  girders lt»7 

Sections  of  lateral  struts 171 

Sections  of  members  for  highway  bridges 228,  220 

Sections  of  meml)ers  of  elevated  railroads 180 

Secti<ms  of  top  chords  and  inclined  end  posts 170 

Sections  of  transverse  struts 171 

Sections  of  vertical  jxjsts 171 

Sections  of  web-stifl'ening  struts 171 

Semi-cantilevers 02 

Sense  of  fitness,  inherent 14 

Sewers,  removal  of 291 

Shafting 205 

Sheared   edges 254 

Shinnning-plates  for  draw-spans 210 

Shipping  metal 201 

Shipping  tiniber 290 

Shipping  weight 252 

Siiipping  weights,  checking  of 280 

Shoeing  j)ile8 278 

Shoe  plates   177 

Slioes  and  end-bearing  rollers  for  draw-spans 209 

Siiop-dravvings   243,  337,  343 

Simple  tnis,s-.spans  erected  by  cantilevering 70 

Simplicity  in  designing 13 

Single  concentrated  loud  method 200 

Sinking  caissons 292 

Sinking  piers 315 

Sinking  piers  into  bed-vock 312 

Sioux  City  Bridge 135 

Sioux  City  Bridge  triangulation 324 

Sizes  of  (Irawings 335,  344 

Sizes  of  i)edestals  for  elevated  railroads 181 

Skew-bridges,  avoidance  of 18 

Skinning  structures 3 

Slabs 245 

Slide-rule,  use  of 333 

Sliding  feet  for  columns 179 

Smooth  track  for  elevated  railroads 101 


: 


INDEX. 


391 


PAQK 

.   296 
8,  342 

.  lay 

17.208 


PAOK 

Some  Disputed  Points  in  Railway-Bridge  Designing. .. .  265 

Spacing  oi  arches 8o 

Spacing  of  girders  for  draw-spans 189 

Spacing  of  girders  in  railroad  bridges 143 

Spacing  of  girders  in  trestles 87 

Spacing  of  l-strut  angles 177 

Spacing  of  joists  in  highway  bridges 214 

Spacing  of  rivets  at  ends  of  cover-plates l(»i) 

Spacing  of  stringers  in  railroad  bridges 143 

Spacing  of  ties  in  elevated  railroads 102 

Spacing  of  tracks  in  railroad  bridges 143 

Spacing  of  trusses  in  railroad  bridges 144 

Spacing-rings  for  rollers  of  draw-spans 197 

Span  lengtlis,  economic 33,  34 

Span  lengths  for  elevated  railroads 92,  93 

Span  lengths  for  trestles 8(5,  180 

Special  investigations  concerning  cantilever  bridges....  (59 

Specialist,  duties  of 3 

Specialist,  necessity  for 3 

Specifications,  filing  of 344,  'M^y 

Specifications  for  cast  iron 2i^'J. 

Specifications  for  cast  steel 253 

Specifications  for  highway  bridges  and  viaducts 213 

Specifications  for  highway  draw-spans 237 

Specifications   for  materials 243 

Specifications  for  piles 277 

Specifications  for  railroad  draw-spans 182 

Specifications  for  railroad  structures 141 

Specifications  for  timber  trestles 27(5 

Specifications  for  wrought  iron 2r)2 

Specifications,  spirit  of 2(54 

Speed  of  testing-machine 247 

Spirit  of  the  specifications 2(54 

Splices  in  colunnis 90.  180 

Splices  in  fianges  of  plate  girders 1(57 

Sjiices  in  members  of  draw-spans 192 

Splices  in  webs  of  plate  girders 16(» 

Stamping-hammers  29S 

Stiiudard  specifications  for  highway  bridges 131 

Star-struts,  objections  to 28 

Star-struts,  tests  of 28 

Station-hcnises  for  elevated  railroads 1(»2 

Stay-plates   174. 232 

Steam-eDgines  for  draw-spans 120,  201 

Steel    buildings .33 1 

Steel  ca)  isons , , 309 


398 


INDEX. 


PAOB 

Steel  piers,  braced 313 

Steel  shells  tilled  with  concrete 310 

Steel  tapes  for  hasc-line  measurements 317 

Still'  bottom  ciiords 171 

Stiff  bottom  chords  in  highway  bridges 220 

Stidcncrs.  crimping  of 5)4 

Stidcncrs  for  drums 1<J3 

iStiffcners  for  girders  over  drums 2(X) 

Stidcning  angles  for  stringers 173 

Stiff  top  chords  for  drawbridges ]S3,  11)1 

Stone  for  masonry,  inspection  of 2!)(j 

Storage- batteries  for  operating  draw-spans 120,  201 

Straiglitening  metal 254 

Strengthening  piers  of  East  Omaha  Jkidge 314 

Stress  diagrams,  contents  of 330 

Stress  diagrams,  styles  of ; 330 

Stresses,  dead-load 333 

Stresses  in  anchor-arms  of  cantilever  bridges 58 

Stresses  in  cantilever-arms 58 

Stresses  in  cantilever  bridges 58 

Stresses  in  main  central  spans  of  cantilever  bridges.  .50,  00 

Stresses  in  suspended  sj)ans  of  cantilever  bridges 58 

Stresses  in  trestle  columns,  combination  of 88 

Strictness  of  inspection 203 

Stringer   bracing 173 

stringers,  projmrtioning  of 172 

Stringers,   timber 274 

Structure,  general  layout  of 330 

Structures  designed  by  manufacturers 3 

Struts  of  two  liU'cd  angles 171 

Study  in  outlining  spans 5(1 

Study  of  testhetics  in  any  bridge-design 4!) 

Styles  of  bridges  for  various  s])an  lengths 217 

Styles  «)f  drawbridges  for  various  span  lengths 182 

Styles  of  highway  draw-.spans 238 

Styles  of  railroad  bridges  for  various  span  lengths 145 

Styles  of  stress  diagrams 330 

Submerged  piers  for  bascule  bridges 100 

Subpunching 250 

Subpunching,  necessity  for !> 

Superelevation  for  wooden  trestles 27!) 

Superelevation  on  curves  of  elevated  railroads $)5 

Superelevation  on  curves  of  railroad  bridges 143 

Superlluous  metal,  use  of 17 

Superiority  of  cantilever  bridges,  imaginary 55 

Superiority  of  Petit  truss 19 


INDEX. 


399 


PAOR 

Supplomonlary  aiifjlos  for  opon- webbed,  riveted  giniers. .  27 

Surplus  material,  removal  of 2{)l 

Suspended  span,  eeonomie  leiif^tn  of 70 

Suspended  spans,  eonneetion  of (59 

Suspended  spans  of  cantilever  bridj^es,  stresses  in oS 

Suspenders  for  cantilever  hridj^cs (»5 

Suspcndeis  for  iiij,'li\vay  bridjjes •22(l 

Suspeiulers  for  railroad  bridfjes 14H 

Suspension  bridges,  lack  of  rigidity  in 57 

Suspension  bridges,  proper  locations  for 'u 

S\vay-l)racing  for  deck-bridges , 21'.) 

Sway-bracing  for  draw-spans 191 

Sway-bracing  for  highway  bridges 219 

Symmetry  in  layout  about  a  middle  ])lane 48,  49 

Symmetry  in  pier  designing ;i()S 

Symmetry  in  riveting,  necessity  for 22 

Symmetry  of  section,  necessity  for 21 

Systemization  of  cantilever-bridge  designing o7 

Systemization  of  knowledge 34(5 

System ization  of  work \'.i 

Table  of  data  for  various  cantilever  bridges 71 

Tape-lines ;}1K 

Tapered  reamers 25(> 

Targets    ,'J25 

Temperature,  ell'ects  of  chajiges  in 155 

Temporary  bracing  for  steel  cylinders ;{12 

Tenders   ' 2(i4 

Tensile  strcnigth 247 

Tension  on  anchor-bolts  in  trestles 148 

Testing  cement 295 

Testing  foundations 29 1 

Testing  full-size  mendiers 288 

Testing-niachiiu\  speed  of '. 247 

Testing,  methods  of 24(1 

Testing  openiting  machinery 289 

Testing  [)aints 10 

Testing  taj)e-linea ;}  1 8 

Test-j)ieces,  annealing  of 247 

Test-pieces,  number  of 249 

Tests  of  angles  connected  by  one  leg  only 27 

Tests  of  full-size  eyebars 25(1 

Tests  of  full-size  meinbers  and  details 253 

Tests  of  rollers  for  draw-sj)ans 25;j 

Tests  of  star-struts 28 

Text-books  on  substructure .301 

Thickness  of  track-segments 195 


400 


INDKX. 


PAGE 

Thinning  paints 2G() 

Tlioroughtures,  closing 202 

Thorouglnifrts  in  inspertion 281 

I'hiouda,  speeificiitions  for 259 

Thrust  transference,  provision  for 20 

Tie-plates 174, 232 

Tie-spacing  for  elevated  railroads 102 

Tiiid)er 201,  293 

Tind)er-b<)lts 142 

Timber  caissons 308 

Timber  conijjrossion-nieiinK'rs 270 

Timber,  defects  in 299 

Tind)er,  e.vtreiiie  fibre-stress  for 150 

Timber  for  trestles 277 

Timber,  griUage 303,  309 

Timber  inspection 297 

'i'indier  inspectors,  reports  of 299 

Timber  piers 314 

Timber  portions  of  highway  l)ridges 214 

'I'imber  j)ortions  of  railroad  slnictiucs 141 

Timber  stringers 274 

Timber  trestles 273 

IMtles  for  drawings 339,  342 

Toggle-links  for  draw  machinery 208 

Toggles  for  semi-cantilevers 62,  03 

Top  chords,  sections  of 170 

Tops  of  columns  in  trestles  and  elevated  railroa<ls 2(» 

Tops  of  trestle  columns 179 

Tor.sion,  avoidance  of 20 

Tower-bracing   179 

Tower-bracing  for  draw-spans 191 

Tower  Bridge  of  London 105 

Towers  for  draw-spans 183 

Towers  of  Halsted  Street  Lift-liridge 108 

'!\)wer-spacing  in  elevated  railroads 149 

Tracing-cloth,  quality  of 340 

Tracing-doth,  working  on 330 

Track-segments  of  drums 194 

Traction-bracing  in  truss-bridges 173 

Traction  load 154 

Train-sheds,  arches  for 79,  81 

Transference  of  thrust,  provision  for 20 

Transferred  load 154 

Transferred  load  for  draw-spans 185 

Transits  for  triangulation 323 

J.ranHversG  struts,  sections  of. , , ...--, Ill 


IVDEX. 


401 


PAGE 
2()() 

202 

281 

259 

2« 

74, 2:}2 
102 

01,21):} 
142 
308 
27(5 
20!» 
150 
277 

lo.s,  :}0!) 

21»7 
,  2i«) 
.  314 
.  214 
.  141 
.  274 
.  273 
339, 342 
.  208 
62,  ()3 
.  170 
.  20 
.  17!) 
.  20 
.  171) 
.  15)1 
.  105 
,  .  1H3 
.  lOH 
.  .  14!) 
.  .  340 
.  .  33(1 
..  194 
..  173 
..  154 
79,81 
..  20 
.  .  154 
.  .  185 
.  .  323 
..  17J 


PAGE 

TriivcllcrH  for  troallos 87 

'I'Kilcd  tiiiihor  for  cU'vuted  railroiulH !»3 

'I'n'iit isc  oil  rt'duiidiint  iiicinlu'is 19 

Tn'mio,  use  of 2{)3 

Ticsl Ic-ldiiciuj;.   \voo(U>n 270 

'ricstlc  (•olumiis.  Itultcr  for 87 

Tn'sllc  coliiiiiiis.  conibiniitioii  of  Htivssps  in 88 

'I'lcstli'  coliiiimH,  (If'tailH  for 87 

Tn-sl  Ics   HO 

Tn'stlcH,   details   for 87 

Tn'stlcs,  t'cniioiiiy  in 30,  84 

'i'r«'stl('-to\vcr   III  acing 179 

Trestle-towers 148 

Trestle-towers,  hraeing  tor 8!) 

Trestle-towers  for  liighwuy  structures 220 

Trestle-work,  inspection  of 293 

Triangulation   317 

Triaiigulation-liubs 320 

Triaiigulation-shoets    325 

Triaiigulation  to  pier-points 325 

Triiiuiiiiif,'  cliaiiiiel-ends 170 

Triuiniing  desij,Mis 2 

Triuiming  idle  cornel  h 254 

True  economy  in  design 30 

True  economy,  necessity  for 15 

Truss  depths,  economic 30 

Truss  deptlis  for  draw-spans 183 

Trusses,  weights  [ter  foot  of 32!>.  330 

TnisHCH  with  parallel  chords,  economy  in 31 

'I'russes  with  ])olygonal  toj)  ( hords,  ((conoinic  depth  of.  .  32 

Tubes,    filing 345 

Tunil)U<'kles,  siiecilieations  for 25S 

Turned  bolts  for  turntables 1!)3 

Turned  bolts,  specilicationa  for 258 

Turntables,  details  of 192 

Turntables  of  draw-spans,  proportioning  of 128 

Twist-drills 250 

Two-angle  struts 171 

Two-rivet  connections,  objections  to 25 

Types  of  arches 80 

I'ltiinate  tensile  strength 247 

Unbalanced  wind-pressure  on  draws 202 

T'niforni  sizes  of  drawings .335 

Ifiiit  weights  of  materials 152 

Unit   working  i)ressure  on  screw-threads 208 

Unnecessary  cantilever  bridges 55, 56 


402 


INDEX. 


PAOK 

Unprotefied  ooiu  rote  j)i('is HlO 

UnsjitiHiiu'ldi  V  coiuliti*)!!-*  of  existing  clevati'd  ruilruadH, 

rcasoiiH  for '21 

I'iisii|)|Mir(c(l  widths  of  jtlatcs  in  foiiiidcssioii 174 

Uplift  for  liij^'liwiiy  dra\\-«i»iiiis,  as-mnuHl 'I'Mi 

r|»lift  loads  for  diMw-spans IHI 

Upward  dead-load  reactions  in  draw-spans 12*2 

list'  of  east-iron  trininiinj,'s  for  deeoraiion ;V2 

l's«'  of  dynamite  in  pier  sinking ;{(),'{ 

Use  of  high  steel S 

Use  of  jndgment  in  designing 15 

Use  of  labor-saving  devices i;{ 

I'se  of  powder  in  quanying 21)7 

Use  of  slide-rule 3'.i',i 

Use  of  superlluous  metal 17 

Use  of  supplementary  angles  for  open-webbod,  riveted 

girders   27 

Use  of  water-jet  in  pile-driving 314 

Vain  etForta  of  engineers  to  compromise  with  grace  by 

ornamentation   41 

Value  of  faculty  of  judgment 14 

Van  Buren  Street  IJascule  J$ridge,  Chicago 10(i 

Variation  in  cost  of  piers  with  s])an  length 3.> 

Variation  in  weight 25 1 

Variation  of  truss- weight  with  s|)an-length 3.'> 

Variation  of  weight  of  lateral  system  with  span-length..  .'<;") 
Variation  of  weight  of  metal  with  length  of  opening  in 

cantilever  bridges 7<> 

Various  tyj)es  of  arches HO 

Vertical  posts,  sections  of 171 

Vertical  sway-bracing 21i> 

Viaducts   ..." ^^i 

Viaducts,  economic  layouts  of H7 

Viaducts,  ornamentation  of A2 

Violation  of  principles  in  designing 4.  o 

Washers,  s|)ecifications  for 2r>s 

Wasbing  drawings  with  benzine .'MfJ 

Waste  of  money  in  designing ."» 

Water-jet,  nae  of,  in  pile-driving.  ..." 314 

Water-power  for  operating  draw-spans 120 

Wearing-floors 2 1 5 

Web  members  of  open-webbed,  riveted  girders,  sectional 

areas  for 27 

Web-plates  at  ends  of  open-webbed,  riveted  girders 100 

Weba  of  drums 103 

Webs  of  girders  for  elevated  railroads 94 


INDKX. 


403 


PAOB 

\V«'l)-si)li('OH  in  (Iniina I!»;{ 

\V('l)-s|)li<»'s  ill  piiih'  jjinlfTs l(i(i 

Well  .sliUViKTs  for  plain  j^irdt'iH KJS 

W'cli-sliUViiiii};  struts,  soctioiis  ot 171 

W'cij^'liiiij,'  rciict  ions  for  dniw  simiis 122 

Wcijrht  of  IJalstfMl  Street,  Lift-Hrid^e 1()!» 

\\«'ijf|it  of  iiiiisomy  ill  iiiielioi-|iiers  of  eiiiiliiever  luidp's.  (i7 

Wei^^'lit  of  metal  ill  aiuiiora^ies  of  c.iiit ilever  hiid<,'es.  ...  I'.i 
\\'<'i<,'lil  of  iiietiil  ill  aiiclior-spaiis  of  cantilever  hridj,'es.  7.S,  74 

W'eij^lit  of  tiiivelier,  elleet   of.  on  erection  stresses (iO 

\\«'i<,dits  of  l)ri(l«,'es  let  by  liiiiip  siini  and  hy  pound  price.  2 

W'eifjhts  of  trestle-aiicliora;^es <M) 

W'eijrlits  per  foot  of  lateral  systems .'$2!> 

W'ei^lits  per  foot  of  trusses ',i2\) 

Wells  ill  anehoin'res  of  eantilev-r  l»rid};es {\(\ 

^^'heel-J(ual•ds  for  iiif^lnvay  I>rid{,'es 215 

Wiiipple  truss,  ohjeelions  to 1!) 

^\  ideiiinj;  cantilever  hridj^'es  over  main  piers (V\ 

\\'iilo\v  mattresspH  around  piers 314 

Wind  loads  for  draw-spans 185 

Wind  loads  for  iiififliuay  hridjjes 224 

Wind  loads  for  liiphway  viaducts 224 

Wind  loads  for  railroad  hridj^es 153 

Wind  stres.ses.  recordinjf  of 334 

\N'inner  Brid<,'e 114 

Wooden  eompressioii-inemlxTs 276 

Wooden  liand-rails  for  liij^hway  l)rid«ifes 215 

Wooden    st rin<.'(  IS 274 

Wooden  trest  le  liraciii<j 27(5 

Work  (lesiij;iied  hy  author's  Hpeeincations 4 

Workin<,'-dra\viiifrs    243,  337 

Workinj,'-pressure  on  screw-threads 208 

Workiiif^-stresse.s,  int(-nsities  of 155 

Workmanship 254 

Workmen 2(i  I 

Wrifjlit's  Desipninfj  of  Draw-Spans 122 

Wrouglit  iron,  specifications  for 252 

Z-bar  columns 90,  95 


